Hillary Clinton unexpectedly left Sunday's 9/11 anniversary ceremony in New York after feeling "overheated," according to her campaign, and her doctor later revealed she was diagnosed with pneumonia Friday.

Her illness led to the cancelation of a scheduled trip Monday and Tuesday to California, according to her campaign.

Clinton spokesman Nick Merrill said in a statement that the Democratic presidential nominee attended Sunday morning's 9/11 ceremony for 90 minutes "to pay her respects" before departing.

"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better," Merrill said, but offered no additional details, including whether the 68-year-old Clinton required medical attention.

Hillary Clinton addressed the media after she left her daughter Chelsea’s New York City apartment shortly before noon. “I feel great,” Clinton said. “It’s a beautiful day in New York.”

Later Sunday, it was revealed Clinton was diagnosed with pneumonia Friday.

"Secretary Clinton has been experiencing a cough related to allergies. On Friday, during follow up evaluation of her prolonged cough, she was diagnosed with pneumonia," Dr. Lisa Bardack said in a statement. "She was put on antibiotics, and advised to rest and modify her schedule. 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.”

Video taken at the scene and posted on social media shows Clinton being held up by three people. She staggers and appears to trip on a curb as they help her into a vehicle.

Clinton's departure from the event was not witnessed by the reporters who travel with her campaign, which did not offer any information about why she left and her whereabouts for more than an hour.

Clinton walked out of daughter Chelsea Clinton's apartment on her own and made only a brief comment to the reporters waiting outside, saying "It's a beautiful day in New York." She waved and posed for a photo with a young girl before getting into her motorcade.

Democratic presidential nominee Hillary Clinton attends the Sept. 11 remembrance ceremony in New York.

The campaign also did not take reporters in the motorcade after Clinton's departure from her daughter's apartment. An aide said the former secretary of state was heading to her home in Chappaqua, New York.

Clinton was scheduled to make a trip to California Monday, but Merrill said Sunday night she will no longer head to the West Coast.

Clinton was supposed to make an appearance on the "Ellen DeGeneres Show" and attend three planned fundraisers in California.

Republican presidential nominee Donald Trump said he didn't know anything about the incident when asked as he left the ceremony.

NEW: Trump tells me "I don't know anything about" Clinton's health incident this morning. #nbc2016 pic.twitter.com/FATsLp8i6e — Ali Vitali (@alivitali) September 11, 2016

Trump supporters have tried to make the case that she's physically unfit for the White House, citing a concussion she sustained in December 2012 after fainting. Her doctor attributed the episode to a stomach virus and dehydration.

Clinton's doctor reported she is fully recovered from the concussion, which led to temporary double vision and discovery of a blood clot in a vein in the space between her brain and skull. Clinton also has experienced deep vein thrombosis, a clot usually in the leg, and takes the blood thinner Coumadin to prevent new clots.

Trump attended the same memorial service at ground zero in lower Manhattan, along with New York's Democratic senators, Chuck Schumer and Kirstin Gillibrand. The weather was warm and humid in New York on Sunday, and there was a breeze at the crowded memorial plaza during the ceremony.

Neither Trump nor Clinton spoke at the event, in keeping with the solemn nature of the annual remembrance of the deadliest terror attack on American soil.

Trump's personal physician has said the Republican presidential nominee is in excellent health both physically and mentally. But the 70-year-old Republican nominee has refused to release his own health records.

Dr. Harold Bornstein's report last December remains the only medical information released so far by the Trump campaign. Bornstein told NBC News he needed just five minutes to write a glowing public assessment of Trump's health as a limousine waited to carry the letter back to Trump.