Hillary Clinton's abrupt exit from a 9/11 memorial service Sunday morning has ignited calls for the Democratic presidential nominee to be more transparent about her health.

In an illness episode her campaign blamed on "overheating," Clinton was escorted to a van before the ceremony was over, appearing to stumble on a curb as she boarded the vehicle. She spent some time at her daughter Chelsea's Manhattan apartment before departing for her home in Chappaqua.

Clinton spokesman Nick Merrill said in a statement that Clinton "felt overheated and so departed to go to her daughter's apartment," but "is feeling much better."

But the incident prompted more calls in the press and among politicians for Clinton to respond to concerns about that episode, as well as an extended coughing fit during a speech on Labor Day.

"I'm a nurse, and I don't want to speculate what went on with Hillary Clinton," Rep. Diane Black, R-Tenn., said on CNN, later adding that "it's probably legitimate to ask what is her health and I think she has to be honest with the American people if she wants to serve the country."

The Washington Post's Chris Cillizza, who had previously chided those who made much of Clinton's coughing fit last week, wrote that Sunday's medical episode made her health a "real issue" in the presidential campaign rather than just a conspiracy theory.

Clinton has fainted in the past, most prominently in 2012, when she suffered a concussion after taking a fall. She also had a blood clot in the brain, which was successfully treated with anticoagulants. Clinton suffers from the relatively common chronic condition hypothyroidism.

Last month, Clinton's doctor reiterated that she's in good health and fit to be president. But that hasn't quieted her opponents, including Republican president nominee Donald Trump, who have been stoking rumors that she isn't disclosing a more serious health problem.

The Trump campaign didn't immediately respond to the incident.