The episode has renewed focus on her health less than two months before an election.
She was diagnosed on Friday, the doctor said, but her condition was not made public until Sunday afternoon. Hours earlier, a video was posted on social media, apparently showing Clinton stumbling and her knees buckling, before being helped by aides into a black van leaving the site of the September 11, 2001 attack in New York.
She was taken to her daughter Chelsea’s home in the city and appeared on her own about two hours later, wearing sunglasses and telling reporters that she was “feeling great.”
The temperature in New York was about 27 degrees Celsius, combined with high humidity.
Clinton wore a high-collared shirt and a dark suit during the memorial honouring the nearly 3,000
people killed in the September 11, 2001 attacks.
Democratic Congressman Joe Crowley told reporters that it was “stiflingly hot”, and he himself had to leave the ceremony.
Clinton’s doctor, Lisa Bardack, said in a statement that the veteran politician has been experiencing a cough related to allergies and that an examination on Friday showed it was pneumonia.
“She was put on antibiotics and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated. I have just examined her and she is now re-hydrated and recovering nicely,” Bardack said.
It was not yet clear whether Clinton would travel to California as planned on Monday to attend campaign and fundraising events.