US Democratic presidential candidate Hillary Clinton leaves her daughter Chelsea's home in New York, United States, Sept 11, 2016, after Clinton left ceremonies commemorating the 15th anniversary of the September 11 attacks feeling "overheated". [Photo/Agencies]

WASHINGTON - Democratic presidential nominee Hillary Clinton said on Monday she could resume campaigning in a "couple of days," after a bout of pneumonia caused her to nearly collapse at an event on Sunday and cancel some of her trips.

Clinton's health scare revived concerns about a tendency toward secrecy that has dogged her campaign, and underscored perennial worries about the medical fitness of candidates for one of the world's most demanding jobs.

"I'm feeling so much better and obviously I should have gotten some rest sooner," she said in an interview on CNN on Monday night, saying she had ignored doctor's orders to rest.

"Well, it will be in the next couple of days. ... I just want to get this over and done with and get back on the trail as soon as possible," she said of her campaign plans.

Clinton, 68, said she had had similar episodes before. "I think really only twice that I can recall. You know, it is something that has occurred a few times over the course of my life. I'm aware of it and usually can avoid it," she said.

Her campaign said it may have been too slow disclosing her pneumonia diagnosis after she nearly fainted at a New York memorial ceremony for the Sept. 11, 2001, attacks. She was diagnosed with the lung infection on Friday.

"I think that in retrospect, we could have handled it better in terms of providing more information more quickly," Clinton spokesman Brian Fallon told MSNBC.

Both Clinton and Republican rival Donald Trump, 70, said they intended to release more of their medical details in the coming days, as their campaigns gear up for the Nov 8 election.