US Democratic party presidential candidate Hillary Clinton kept her pneumonia diagnosis from most of her staff, according to reports, choosing to tell only family and close aides.
Mrs Clinton was reportedly concerned that news of the illness would be exploited by her opponents.
She was diagnosed on Friday and advised to rest, but continued campaigning.
Her team was forced to go public with the diagnosis on Sunday when she left a 9/11 memorial early, appearing weak.
Speaking to CNN late on Monday, Mrs Clinton said she was feeling "so much better".
She also told the broadcaster that she had not disclosed her pneumonia diagnosis, saying: "I just didn't think it was going to be that big a deal."
The US presidential candidate said she had ignored a doctor's "wise" advice to rest for five days. She said she hoped to be back on the trail in "the next couple of days".
The Democratic nominee acknowledged she had lost her balance during Sunday morning's health scare, but said she did not faint.
"I felt dizzy and I did lose my balance for a minute, but once I got in (the van), once I could sit down, once I could cool off, once I had some water, I immediately started feeling better," she said.
Some critics questioned why this had only been revealed after Sunday's events and Mrs Clinton's communications director conceded the incident had been poorly managed.
She will be releasing new medical records to help ease concerns about her health, a campaign spokesman said.
Don Fowler, a former chairman of the Democratic National Committee (DNC), urged the party on Sunday to immediately come up with a process to choose a potential successor for Mrs Clinton, in case she is forced to retire with ill health.
Mr Fowler said he expected Mrs Clinton to fully recover but said taking precautions was necessary.
Ed Rendell, another former DNC chair, told Politico: "There is absolutely no chance Hillary Clinton will withdraw from running for the presidency."
It capped a difficult weekend for Mrs Clinton, who came under attack from Mr Trump for calling half of his supporters "deplorable" people on Friday.
On Monday, the Republican presidential nominee wished her a speedy recovery. He also pledged to release the results of a medical examination he took over the past week.
Mr Trump has repeatedly suggested Mrs Clinton is unfit, telling supporters last month she "lacks the mental and physical stamina" to serve as president.
Other campaign developments
- Donald Trump accused US Federal Reserve Chair Janet Yellen of keeping interest rates low to boost President Barack Obama's popularity. He said she should be "ashamed"
- The Republican candidate also released a new ad attacking Mrs Clinton for calling half of his supporters "a basket of deplorables" last week. "You know what's deplorable? Hillary Clinton viciously demonising hard-working people like you," a narrator says
- Green Party candidate Jill Stein says she would not have killed Osama Bin Laden but instead would have put him on trial
Major fall or just a stumble? - Anthony Zurcher, BBC News
Will Hillary Clinton's near collapse during Sunday's 9/11 memorial ceremonies make her health a political Achilles heel?
The Trump campaign managed to hold its tongue as the news unfolded, probably sensing that the story was damaging enough on its own.
At 68, Mrs Clinton is particularly susceptible to concerns about her health (as is 70-year-old Mr Trump).
But if she returns to her normal campaign schedule without incident, she will go a long way towards putting health concerns to bed.
What is walking pneumonia?
Mrs Clinton's team said she is suffering with "walking pneumonia" - a not-too-serious form of the lung infection which leaves patients feeling unwell but doesn't usually require bed rest or hospital care.
Pneumonia is essentially an infection of the lungs which causes inflammation in the air sacs and fills them with fluid. Symptoms can include a cough, fever, fatigue, chills and shortness of breath.
Anyone can contract pneumonia, although smokers, older people, and sufferers of chronic lung diseases are at increased risk. There are two types - bacterial or viral. Bacterial pneumonia is common and easily treated with antibiotics.
Most people with so-called "walking pneumonia" can recover within a few days. Those with weak immune systems or existing conditions can take weeks to recover, and pneumonia can in some cases be fatal.