New York - Hillary Clinton, the Democratic presidential nominee, has been ill with pneumonia, her doctor revealed in a Sunday evening statement that may shake up the presidential race.

Clinton’s health had been the subject of much speculation, fuelled most recently by a coughing fit she had at the start of a campaign event in Cleveland, Ohio, last Monday.

But now it seems that the symptoms she blamed on seasonal allergies may have been a sign of the pneumonia that was diagnosed on Friday.

Dr Lisa Bardack released a statement via the campaign revealing the diagnosis on Sunday evening, hours after the Democratic presidential nominee was seen stumbling on video after exiting a 9/11 memorial event early.

Bardack said Clinton had an examination at her home in Chappaqua, New York, after the incident, which was a result of dehydration and overheating. She said the candidate is "re-hydrated and recovering nicely."

But the campaign has been criticised by the media for failing to keep the press appraised of her health and for departing from the 9/11 memorial without any press in tow.

A campaign official said that she cancelled her trip to California, to which she was scheduled to leave on Monday morning, because of her illness. She was set to appear at events in both San Francisco and Los Angeles on Monday and Tuesday, and a rally in Las Vegas on Wednesday.

"Sunday's events at the Sept 11 memorial show the importance of having a group of reporters on standby, and close at hand, to cover the potential next president of the United States," Anne Gearan, the Washington Post political reporter and current chair of the Clinton press pool, told CNN.

The Washington Post’s Chris Cillizza, who dismissed what he called “conspiracy theories” about Clinton’s health last week, said he changed his thinking Sunday.

“Whereas Clinton and her campaign could laugh off questions about her health before today, the ‘overheating’ episode makes it almost impossible for them to do so,” he wrote. “Not only has it come at a time when there was growing chatter — with very little evidence — that her health was a problem but it also happened at a 9/11 memorial event — an incredibly high-profile moment with lots and lots of cameras and reporters around.”

Questions about Clinton's health have swirled in recent weeks as Republican rival Donald Trump frequently questioned her stamina on the campaign trail. Trump said in an interview last week after Clinton’s coughing fit that he would release his full medical records if Clinton makes her records public.

He was reportedly told about his Democratic rival having to leave the 9/11 memorial early, according to a Washington Post reporter, but did not comment on Sunday.

On Friday, the same day she was diagnosed with pneumonia, Clinton had a packed schedule, according to Bloomberg reporter Jennifer Epstein. She appeared at two fundraisers, a long national security meeting and a CNN interview.

But while many on social media questioned whether the illness would have been disclosed if the Sunday incident hadn't occurred, other were defending Clinton.

Former Michigan Governor Jennifer Granholm, a Clinton supporter, said "'powering through" illness is what women do," in a tweet.

"I am glad to learn that Secretary Clinton is already feeling better and I wish her a speedy recovery," interim Democratic National Committee Chairwoman Donna Brazile said, according to Bloomberg. ”I look forward to seeing her back out on the campaign trail and continuing on the path to victory.”

The campaign said late Sunday night it was cancelling a trip to California starting Monday for fundraisers and an appearance on "Ellen."

Videos spread quickly online showing Clinton stumbling into a black van before leaving a memorial event early in New York City on Sunday. Aides said Clinton was feeling "overheated" and left to rest at her daughter's apartment.

Clinton emerged about 90 minutes later, telling reporters she felt great. She then was driven to her home in Chappaqua, NY, where her doctor performed an examination.