The incident at New York's Ground Zero, in which the 68-year-old Clinton seemed to buckle and lose her footing as she was helped into her vehicle, offered Republican Donald Trump a new opening to attack his White House rival with just 15 days before their first high-pressure presidential debate.
Clinton had been seeking to bounce back from a blunder Friday, when she told donors that half of Trump's supporters belonged in a "basket of deplorables" -- so Sunday's episode was certainly ill-timed.
The former secretary of state spent 90 minutes at the ceremony in lower Manhattan, greeting some relatives of those killed in the terror strikes 15 years ago, her campaign said in a statement.
"During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better," it added.
Later, the campaign released a statement from her personal doctor, Lisa Bardack, who revealed that Clinton had been diagnosed with pneumonia Friday and was suffering from dehydration.
"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," Bardack said.
"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."
A campaign aide said Clinton had been examined at her home in Chappaqua, New York, after leaving Ground Zero.
A video posted on Twitter showed Clinton appearing unsteady as she waited to get into a black van to leave the 9/11 service.
She appeared to stumble as she was helped into the vehicle, and had to be held up on either side by members of her entourage.
It was a humid day in New York, with temperatures around 82 degrees Fahrenheit (28 Celsius).
"I didn't see anything unusual. What I can tell you is that many of us (had) a sigh of relief when a gust of wind would come by because it was incredibly stifling," Democratic congressman Joe Crowley, who stood near Clinton for about an hour at the ceremony, told MSNBC.
Clinton appeared better as she walked out of her daughter Chelsea's home a few hours later, smiling and posing for pictures with a young girl before departing for Chappaqua.
"I'm feeling great, it's a beautiful day in New York," Clinton said.
But a senior aide said a planned fundraising trip to California on Monday was "currently in discussion," as the campaign mulls Clinton's next steps in the heat of a narrowing race.
Trump -- who also attended the 9/11 ceremony -- was uncharacteristically silent on Twitter about Clinton's illness, as both took a break from formal campaigning to mark the somber day.
But the businessman, his spokespeople and surrogates have promoted the idea in recent weeks that Clinton has serious health problems.
The internet is awash with claims that she may have a brain tumor, Parkinson's or dementia.
Trump, 70, has said Clinton is "not strong enough to be president" and that she "lacks the mental and physical stamina to take on ISIS" jihadists.
The root of the claims lies in 2012, when Clinton was nearing the end of her State Department tenure. A stomach virus and dehydration prompted her to faint, causing what her doctor said was a concussion.
They said they found a blood clot on the brain and Clinton temporarily suffered from double vision. She later received the all-clear.
The former first lady has dismissed "conspiracy theories" about her health and pointed to a letter from her doctor declaring her fit to serve as president.
A Monday coughing spell by Clinton prompted renewed questions about whether she is physically fit for the job.
'Vitality And Viability'
Clinton's illness is unlikely to be a turning point in the race, said Jennifer Lawless, a professor of government at American University in Washington.
"What the Clinton campaign needs to do over the course of the next several days is demonstrate her vitality and viability. She has to be at tons of events and seem very energetic," Lawless told AFP.
That strategy could be in jeopardy if Clinton follows her doctor's recommendation to "rest and modify her schedule."
Larry Sabato, a veteran political scientist at the University of Virginia, said Clinton's team should release a full health record.
"We really haven't gotten very much, essentially a letter from her doctor," he told CNN, but added that Trump should be held to the same requirement.
A Washington Post-ABC News poll out Sunday shows Clinton leading Trump 46 percent to 41 percent among likely voters.
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