Hillary Clinton has said she became dizzy but did not lose consciousness when she stumbled on the way out of the 9/11 memorial ceremony on Sunday.
The Democratic presidential candidate said in an interview on CNN that she felt dizzy and "did lose my balance" while waiting for the motorcade. But she felt immediately better after getting into her air-conditioned vehicle.
She said she did not immediately disclose that she has been diagnosed with pneumonia because she "just didn't think it was going to be that big a deal".
Mrs Clinton said her doctor had told her to rest for five days, but she "didn't follow that very wise advice".
She added that she will be back on the campaign trail "in the next couple of days".
Asked whether she had fainted at the New York event, Mrs Clinton replied: "No, I didn't. I felt dizzy and I did lose my balance for a minute. But I got in, once I could sit down, once I could cool off, once I got some water, I immediately started feeling better."
She later told supporters via text message and Facebook: "I'm feeling fine and getting better." She added: "Like anyone who's ever been home sick from work, I'm just anxious to get back out there."
Her comments came as her campaign scrambled to head off lasting damage from a difficult weekend. Aides are promising to release more of her medical records following her bout of pneumonia and conceding they were too slow in providing information about her condition.
An outbreak of respiratory illness swept through Mrs Clinton's campaign in the weeks before she was diagnosed with pneumonia, campaign aides said.
They said it was Mrs Clinton's decision to attend weekend fundraisers and the Sunday ceremony in lower Manhattan, despite the diagnosis of pneumonia.
Late on Sunday evening, she cancelled a California campaign trip scheduled for early this week. Her husband, former president Bill Clinton, will headline some of her events.
Her Republican Party rival Donald Trump, who has long questioned Mrs Clinton's fitness for the presidency, was notably restrained on the health matter, saying he hopes she feels better soon.
He focused on her remark last week that half of his supporters belonged in "a basket of deplorables," denouncing the comment as "an explicit attack on the American voter" and suggesting that it makes her unfit for the presidency.
"You cannot run for president if you have such contempt in your heart for the American voter," he said at a rally in Asheville, North Carolina. "You can't lead this nation if you have such a low opinion of its citizens."
But even as the tycoon defended his backers, one lashed out at protesters in the hall by appearing to punch and slap them.
The rally was interrupted several times by demonstrators and, at one moment, brief violence.
As several protesters were being escorted out by security, a man in the crowd grabbed a male protester around the neck and then punched him. He then slapped at a woman being led out. The Trump supporter was not ejected by security.
The celebrity businessman talked through the scuffle and said after the disturbance: "Is there any place more fun than a Trump rally?"