"I was supposed to rest five days -- that's what they told me on Friday -- and I didn't follow that very wise advice," EFE news quoted Clinton as saying during a phone interview to CNN.
The 68-year-old Clinton said that although she was "feeling so much better", she had decided to listen to her doctors and take a few days off from campaigning to get well.
The Clinton campaign has been criticised for being slow to reveal where the candidate was and the state of her health.
Clinton defended her campaign's handling of the medical episode, saying it "met a high standard of transparency" about her health.
The Democratic presidential candidate, who is resting at her Chappaqua, New York, house, said she did not think the pneumonia was "going to be that big a deal".
"What happened ... was that I just was incredibly committed to being at the memorial -- as a senator on 9/11, this is incredibly personal to me," Clinton said.
The Democratic presidential candidate said the high temperatures and humidity in Lower Manhattan affected her and that she lost her balance on Sunday.
"I could feel how hot and humid it was. I felt overheated. I decided that I did need to leave. And as soon as I got into the air-conditioned van, I cooled off, I got some water, and very quickly, I felt better," Efe news quoted Clinton as saying.
The Democratic presidential candidate cancelled a fundraising trip to California scheduled for Monday and Tuesday, as well as the taping of an appearance on the "Ellen" show, hosted by actress Ellen DeGeneres.
Clinton's rival, Republican Donald Trump, has mostly avoided discussing her medical episode.
On Monday, Trump said he hoped Clinton would recover quickly from her bout with pneumonia, adding that he underwent a physical check-up last week and planned to release the results soon.
"Something's going on, but I just hope she gets well and gets back on the trail," Trump told Fox News.
This website uses cookies.