(CNN) Hillary Clinton has pneumonia, her doctor said Sunday, hours after the Democratic nominee stumbled and exited a 9/11 commemoration ceremony early.

The incident seems certain to prompt further scrutiny of Clinton's health and her campaign's transparency -- though Republican rival Donald Trump was uncharacteristically silent throughout a solemn day marking the 15th anniversary of the September 11 attacks.

Clinton, 68, was diagnosed on Friday with pneumonia, and "was put on antibiotics, and advised to rest and modify her schedule," Dr. Lisa Bardack said in a statement.

"While at this morning's event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely," said Bardack, chairman of internal medicine at the Mount Kisco Medical Group.

Bardack attended to Clinton at her Chappaqua home Sunday.

Campaign spokesman Nick Merrill said Clinton "continues to feel better" Monday morning.

The Clinton campaign declined to answer questions Sunday about why her diagnosis wasn't disclosed Friday. Many people even inside the Clinton campaign did not know about the pneumonia diagnosis Friday. An aide said that Clinton "thought she could push through this ... she was feeling better" -- until Sunday morning.

The episode quickly impacted Clinton's campaign. Late Sunday night a campaign spokesman said a Clinton trip to California was being scrubbed, including several high-dollar fundraisers and television appearances by the candidate. Clinton donors who had planned to attend the San Francisco fundraiser Monday were told by the campaign that the event would go on as scheduled and that attendees could expect to hear from the candidate "via teleconference."

Instead Clinton now plans to stay home and prepare for the upcoming debates, while her advisers to discuss how to move beyond arguably the worst weekend of the campaign.

Two people close to the campaign said Monday that Clinton will almost certainly have to release more medical records -- or do an interview on her health, specifically. She is likely to resume campaigning Wednesday, but will almost certainly not fly to Nevada, as originally planned. She is more likely to give her economic-themed speech in a battleground state far closer to New York: Pennsylvania, New Hampshire and Ohio are possibles.

Clinton's health came into sharp focus Sunday as she appeared wobbly and stumbled as Secret Service agents helped her into a van to exit a memorial service marking the 15th anniversary of the September 11, 2001, attacks. The Democratic nominee traveled to her daughter's nearby apartment and the campaign initially said she felt "overheated."

Hillary Clinton 9/11 NYC pic.twitter.com/q9YnsjTxss — Zdenek Gazda (@zgazda66) September 11, 2016

A video of Clinton's departure from the 9/11 event at Ground Zero in Manhattan, posted by Twitter user Zdenek Gazda, shows her clearly stumbling, with Secret Service agents helping her into the van in the motorcade.

Clinton appeared to faint as she was leaving the Ground Zero ceremony, two law enforcement officials -- including one who witnessed the incident -- told CNN.

Leaving her daughter's apartment a little before noon, Clinton told reporters she was "feeling great."

"It's a beautiful day in New York," she added, while smiling and waving to supporters on the street before getting into her motorcade.

On Sunday night campaign spokesman Nick Merrill said a planned California campaign trip Monday and Tuesday has been scrubbed.

Other incidents

Clinton suffered a two-minute coughing fit during an event in Cleveland last week, which she blamed on allergies -- and wryly put on the shoulders of GOP nominee Donald Trump.

"Every time I think about Trump I get allergic," she joked in front of the crowd.

Trump and his top surrogates have claimed that she faces a health crisis, citing selectively edited videos of public events to advance claims she's suffered seizures.

There is no credible evidence to support any of these claims. Clinton's physician -- the only person to speak on the record who has actually examined her -- has repeatedly affirmed the former secretary of state's health and fitness for the highest office in the land.

In an interview on "Fox News Sunday" in August, former New York City mayor and Trump surrogate Rudy Giuliani first accused the mainstream media of hiding evidence, then encouraged doubters to "go online and put down 'Hillary Clinton illness.' "

During an appearance last month on the Jimmy Kimmel show, Clinton called the GOP claims about her health a "wacky strategy."

"I don't know why they are saying this," she said. "I think on the one hand, it is part of the wacky strategy, just say all these crazy things and maybe you can get some people to believe you."

Trump camp muted

Trump responded to the incident for the first time on Monday morning, but his initial comments were relatively muted for the Republican nominee who has repeatedly brought up Clinton's health and stamina on the campaign trail.

"I really just don't know. I hope she gets well soon," Trump told Fox News in an interview when asked about the incident, adding later, "Something is going on but I just hope she gets well and gets back on the trail, and we'll be seeing her at the debate." Trump also said he would be releasing to the public results from a doctor's physical he received last week.

Throughout Sunday Trump and surrogates were quiet about Clinton's health. Sources involved in Trump's campaign said they want to be respectful about the health of his Democratic rival.

One said the campaign's message was to wish Clinton well and hope she gets the necessary medical care.

Another source said surrogates have been instructed to be respectful and not to post on social media about Clinton's health incident.

Separately someone familiar with the situation said staff was instructed to keep quiet and not post anything negative on social media about the issue -- with termination a potential consequence for doing so.

Trump himself has not tweeted about Clinton's health Sunday, and this afternoon while he visited a firehouse he refused to comment on her health. "I don't know anything about it, don't know anything about it," he told reporters.

Trump's running mate, Indiana Gov. Mike Pence, refused to acknowledge reporters' questions Sunday about Clinton and her health at an awards ceremony that he presided over in Indianapolis.

Green Party presidential candidate Dr. Jill Stein issued a statement Monday wishing Clinton the best.

"In this extraordinary campaign cycle, where the stakes are so high and the media can be merciless, it can be easy to forget that presidential candidates are human beings too," Stein said.

Health history

The most recent major health problem Clinton has suffered was a scare in 2012 following a concussion that resulted from a fainting spell -- but she is in good health now, according to a statement from her doctor that Clinton's campaign released in July 2015.

"Mrs. Clinton is a healthy 67-year old female whose current medical conditions include hypothyroidiam and seasonal pollen allergies," wrote Dr. Lisa Bardack, Clinton's doctor since 2001, in a health statement provided by Clinton's campaign.

The health statement says the blood clot, also known as a thrombosis, that Clinton suffered between her brain and skull following a fainting spell and concussion in late 2012 is completely resolved.

Photos: Hillary Clinton's life in the spotlight Photos: Hillary Clinton's life in the spotlight Hillary Clinton accepts the Democratic Party's nomination for president at the Democratic National Convention in Philadelphia on July 28, 2016. The former first lady, U.S. senator and secretary of state was the first woman to lead the presidential ticket of a major political party. Hide Caption 1 of 43 Photos: Hillary Clinton's life in the spotlight Before marrying Bill Clinton, she was Hillary Rodham. Here she attends Wellesley College in Massachusetts. Her commencement speech at Wellesley's graduation ceremony in 1969 attracted national attention. After graduating, she attended Yale Law School. Hide Caption 2 of 43 Photos: Hillary Clinton's life in the spotlight Rodham was a lawyer on the House Judiciary Committee, whose work led to impeachment charges against President Richard Nixon in 1974. Hide Caption 3 of 43 Photos: Hillary Clinton's life in the spotlight In 1975, Rodham married Bill Clinton, whom she met at Yale Law School. He became the governor of Arkansas in 1978. In 1980, the couple had a daughter, Chelsea. Hide Caption 4 of 43 Photos: Hillary Clinton's life in the spotlight Arkansas' first lady, now using the name Hillary Rodham Clinton, wears her inaugural ball gown in 1985. Hide Caption 5 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons celebrate Bill's inauguration in Little Rock, Arkansas, in 1991. He was governor from 1983 to 1992, when he was elected President. Hide Caption 6 of 43 Photos: Hillary Clinton's life in the spotlight Bill Clinton comforts his wife on the set of "60 Minutes" after a stage light broke loose from the ceiling and knocked her down in January 1992. Hide Caption 7 of 43 Photos: Hillary Clinton's life in the spotlight In June 1992, Clinton uses a sewing machine designed to eliminate back and wrist strain. She had just given a speech at a convention of the International Ladies' Garment Workers Union. Hide Caption 8 of 43 Photos: Hillary Clinton's life in the spotlight During the 1992 presidential campaign, Clinton jokes with her husband's running mate, Al Gore, and Gore's wife, Tipper, aboard a campaign bus. Hide Caption 9 of 43 Photos: Hillary Clinton's life in the spotlight Clinton accompanies her husband as he takes the oath of office in January 1993. Hide Caption 10 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons share a laugh on Capitol Hill in 1993. Hide Caption 11 of 43 Photos: Hillary Clinton's life in the spotlight Clinton unveils the renovated Blue Room of the White House in 1995. Hide Caption 12 of 43 Photos: Hillary Clinton's life in the spotlight Clinton waves to the media in January 1996 as she arrives for an appearance before a grand jury in Washington. The first lady was subpoenaed to testify as a witness in the investigation of the Whitewater land deal in Arkansas. The Clintons' business investment was investigated, but ultimately they were cleared of any wrongdoing. Hide Caption 13 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons hug as Bill is sworn in for a second term as President. Hide Caption 14 of 43 Photos: Hillary Clinton's life in the spotlight The first lady holds up a Grammy Award, which she won for her audiobook "It Takes a Village" in 1997. Hide Caption 15 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons dance on a beach in the U.S. Virgin Islands in January 1998. Later that month, Bill Clinton was accused of having a sexual relationship with former White House intern Monica Lewinsky. Hide Caption 16 of 43 Photos: Hillary Clinton's life in the spotlight Clinton looks on as her husband discusses the Monica Lewinsky scandal in the Roosevelt Room of the White House on January 26, 1998. Clinton declared, "I did not have sexual relations with that woman." In August of that year, Clinton testified before a grand jury and admitted to having "inappropriate intimate contact" with Lewinsky, but he said it did not constitute sexual relations because they had not had intercourse. He was impeached in December on charges of perjury and obstruction of justice. Hide Caption 17 of 43 Photos: Hillary Clinton's life in the spotlight The first family walks with their dog, Buddy, as they leave the White House for a vacation in August 1998. Hide Caption 18 of 43 Photos: Hillary Clinton's life in the spotlight President Clinton makes a statement at the White House in December 1998, thanking members of Congress who voted against his impeachment. The Senate trial ended with an acquittal in February 1999. Hide Caption 19 of 43 Photos: Hillary Clinton's life in the spotlight Clinton announces in February 2000 that she will seek the U.S. Senate seat in New York. She was elected later that year. Hide Caption 20 of 43 Photos: Hillary Clinton's life in the spotlight Clinton makes her first appearance on the Senate Environment and Natural Resources Committee. Hide Caption 21 of 43 Photos: Hillary Clinton's life in the spotlight Sen. Clinton comforts Maren Sarkarat, a woman who lost her husband in the September 11 terrorist attacks, during a ground-zero memorial in October 2001. Hide Caption 22 of 43 Photos: Hillary Clinton's life in the spotlight Clinton holds up her book "Living History" before a signing in Auburn Hills, Michigan, in 2003. Hide Caption 23 of 43 Photos: Hillary Clinton's life in the spotlight Clinton and another presidential hopeful, U.S. Sen. Barack Obama, applaud at the start of a Democratic debate in 2007. Hide Caption 24 of 43 Photos: Hillary Clinton's life in the spotlight Obama and Clinton talk on the plane on their way to a rally in Unity, New Hampshire, in June 2008. She had recently ended her presidential campaign and endorsed Obama. Hide Caption 25 of 43 Photos: Hillary Clinton's life in the spotlight Obama is flanked by Clinton and Vice President-elect Joe Biden at a news conference in Chicago in December 2008. He had designated Clinton to be his secretary of state. Hide Caption 26 of 43 Photos: Hillary Clinton's life in the spotlight Clinton, as secretary of state, greets Russian Prime Minister Vladimir Putin during a meeting just outside Moscow in March 2010. Hide Caption 27 of 43 Photos: Hillary Clinton's life in the spotlight The Clintons pose on the day of Chelsea's wedding to Marc Mezvinsky in July 2010. Hide Caption 28 of 43 Photos: Hillary Clinton's life in the spotlight In this photo provided by the White House, Obama, Clinton, Biden and other members of the national security team receive an update on the mission against Osama bin Laden in May 2011. Hide Caption 29 of 43 Photos: Hillary Clinton's life in the spotlight Clinton checks her Blackberry inside a military plane after leaving Malta in October 2011. In 2015, The New York Times reported that Clinton exclusively used a personal email account during her time as secretary of state. The account, fed through its own server, raises security and preservation concerns. Clinton later said she used a private domain out of "convenience," but admits in retrospect "it would have been better" to use multiple emails. Hide Caption 30 of 43 Photos: Hillary Clinton's life in the spotlight Clinton arrives for a group photo before a forum with the Gulf Cooperation Council in March 2012. The forum was held in Riyadh, Saudi Arabia. Hide Caption 31 of 43 Photos: Hillary Clinton's life in the spotlight Obama and Clinton bow during the transfer-of-remains ceremony marking the return of four Americans, including U.S. Ambassador Christopher Stevens, who were killed in Benghazi, Libya, in September 2012. Hide Caption 32 of 43 Photos: Hillary Clinton's life in the spotlight Clinton ducks after a woman threw a shoe at her while she was delivering remarks at a recycling trade conference in Las Vegas in 2014. Hide Caption 33 of 43 Photos: Hillary Clinton's life in the spotlight Clinton, now running for President again, performs with Jimmy Fallon during a "Tonight Show" skit in September 2015. Hide Caption 34 of 43 Photos: Hillary Clinton's life in the spotlight Clinton testifies about the Benghazi attack during a House committee meeting in October 2015. "I would imagine I have thought more about what happened than all of you put together," she said during the 11-hour hearing. "I have lost more sleep than all of you put together. I have been wracking my brain about what more could have been done or should have been done." Months earlier, Clinton had acknowledged a "systemic breakdown" as cited by an Accountability Review Board, and she said that her department was taking additional steps to increase security at U.S. diplomatic facilities. Hide Caption 35 of 43 Photos: Hillary Clinton's life in the spotlight U.S. Sen. Bernie Sanders shares a lighthearted moment with Clinton during a Democratic presidential debate in October 2015. It came after Sanders gave his take on the Clinton email scandal. "The American people are sick and tired of hearing about the damn emails," Sanders said. "Enough of the emails. Let's talk about the real issues facing the United States of America." Hide Caption 36 of 43 Photos: Hillary Clinton's life in the spotlight Clinton is reflected in a teleprompter during a campaign rally in Alexandria, Virginia, in October 2015. Hide Caption 37 of 43 Photos: Hillary Clinton's life in the spotlight Clinton walks on her stage with her family after winning the New York primary in April. Hide Caption 38 of 43 Photos: Hillary Clinton's life in the spotlight After Clinton became the Democratic Party's presumptive nominee, this photo was posted to her official Twitter account. "To every little girl who dreams big: Yes, you can be anything you want -- even president," Clinton said. "Tonight is for you." Hide Caption 39 of 43 Photos: Hillary Clinton's life in the spotlight Obama hugs Clinton after he gave a speech at the Democratic National Convention in Philadelphia. The president said Clinton was ready to be commander in chief. "For four years, I had a front-row seat to her intelligence, her judgment and her discipline," he said, referring to her stint as his secretary of state. Hide Caption 40 of 43 Photos: Hillary Clinton's life in the spotlight Clinton arrives at a 9/11 commemoration ceremony in New York on September 11. Clinton, who was diagnosed with pneumonia two days before, left early after feeling ill. A video appeared to show her stumble as Secret Service agents helped her into a van. Hide Caption 41 of 43 Photos: Hillary Clinton's life in the spotlight Clinton addresses a campaign rally in Cleveland on November 6, two days before Election Day. She went on to lose Ohio -- and the election -- to her Republican opponent, Donald Trump. Hide Caption 42 of 43 Photos: Hillary Clinton's life in the spotlight After conceding the presidency to Trump in a phone call earlier, Clinton addresses supporters and campaign workers in New York on Wednesday, November 9. Her defeat marked a stunning end to a campaign that appeared poised to make her the first woman elected US president. Hide Caption 43 of 43

"She had follow-up testing in 2013, which revealed complete resolution of the effects of the concussion as well as total dissolution of the thrombosis. Mrs. Clinton also tested negative for all clotting disorders," Bardack writes.

But as a precaution, Clinton remains on daily blood thinners, something her husband previously revealed. Clinton takes Coumadin, the brand name for the blood thinner warfarin. She also takes Armour Thyroid for her hypothyroidism, antihistamines and Vitamin B12.

Clinton has no known drug allergies, does not smoke, use tobacco products or illicit drugs and drinks occasionally, according to Bardack, who also details her exercise regime: yoga, swimming, walking and weight training.

Clinton is up to date on health tests, including a colonoscopy and a few other tests not often seen in a presidential candidate's health disclosure: gynecologic exam, mammogram and breast ultrasound.

She received her most recent physical exam on March 21, 2015, according to the the statement. Her blood pressure was 100/65. Her heart rate was 72 and her EKG was normal. Her total cholesterol was 195, with an LDL ("bad cholesterol") of 118, and an

HDL ("good cholesterol") of 64 and triglycerides of 69.

"She is in excellent physical condition and fit to serve as President of the United States," Bardack's letter concludes.

This story has been updated.