NEW YORK (CBSNewYork/AP) — Hillary Clinton’s doctor revealed the Democratic presidential nominee has been battling pneumonia after she left the 9/11 memorial ceremony in lower Manhattan early due to feeling overheated.

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“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 R. 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.”

Clinton campaign spokesperson Nick Merrill said she is “feeling much better.”

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“Secretary Clinton attended the September 11th Commemoration Ceremony for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen. During the ceremony, she felt overheated so departed to go to her daughter’s apartment, and is feeling much better.”

Despite feeling better, Clinton has canceled her Monday fundraising trip to California.

CBS2’s Steve Langford reported Clinton was seen on West Street, appearing to lean against a post, while being supported by an aide. As a Secret Service agent begins to usher Clinton toward her van, she falters and then, with the assistance of a second agent, appears to collapse, before being surrounded by security and staff.

The 20-second clip shows Clinton’s knees buckling as she stepped towards a van, 1010 WINS reported.

Sources briefed on the incident told CBS2  said that as Clinton was being helped into her vehicle she appeared to have fainted.

Clinton later said she was “feeling great” after having to leave the ceremony. She walked out of her daughter Chelsea’s apartment in Manhattan just before noon on Sunday.

“It’s a beautiful day in New York. I’m feeling great,” Clinton said. “Thank you.”

CBS2’s Dr. Max Gomez said Clinton appeared fine a couple of hours after the disturbing video.

“If you’ve got allergies, you’re being treated with antihistamines, that can dehydrate you,” Gomez said. “If you’ve got pneumonia, and you’re being treated with antibiotics, those can indirectly lead to dehydration.”

Trump, who attended the same event marking the 15th anniversary of the 9/11 attacks, was noticeably restrained. Asked by a reporter about Clinton’s health incident, Trump said, “I don’t know anything.”

The incident compounds an already difficult stretch for Clinton as the presidential race enters its final stretch. Despite Trump’s numerous missteps, the race remains close and many Americans view Clinton as dishonest and untrustworthy.

New York Democratic congressman Rep. Joe Crowley, who stood behind Hillary Clinton at a Sept. 11 memorial ceremony, says he chatted with the Democratic candidate beforehand and that “she did not seem out of the ordinary at all” although the conditions were “stiflingly hot.”

In a telephone interview, Crowley told the Associated Press that at one point, he saw what he thought was a staff member tap Clinton on her shoulder. The person “said something like, “We’re going to move now,”‘ Crowley said.

Sen. Charles Schumer, D-N.Y., echoed Crowley’s sentiments.

“Hillary and I chatted for quite a while about our remembrances of 9/11/01 and our families. It was pretty hot out there, but she seemed fine to me, and left on her own accord,” Schumer told CBS2.

Crowley said that despite the temperate weather outside, heat was an issue for those standing on the cobblestones at the ceremony. He said he was sweating through his shirt.

“We were begging for some kind of wind,” Crowley added.

On Friday, Clinton told donors that “half” of rival Donald Trump’s supporters are in a “basket of deplorables” — a comment that drew sharp criticism from Republicans. Clinton later said she regretted applying that description to “half” of Trump’s backers, but stuck by her assertion that the GOP nominee has given a platform to “hateful views and voices.”

Now Clinton is sure to face new questions about whether she’s physically fit for the presidency. Trump and his supporters have been hinting at potential health issues for months, questioning Clinton’s stamina when she takes routine days off the campaign trail and reviving questions about a concussion she sustained in December 2012 after fainting. Her doctor attributed that 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.

Clinton spent about 90 minutes at the 9/11 event Sunday, standing alongside numerous other dignitaries, including 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.

Trump’s personal physician has said the Republican presidential nominee is in excellent health both physically and mentally. But the 70-year-old 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.

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