In video posted online, Secretary Hillary Clinton is seen being held up by aides, and wobbling as she left the 9/11 memorial ceremony Sunday.
Her campaign says she was feeling "overheated."
It was also revealed she was diagnosed with pneumonia on Friday.
Internal medicine doctor David Targan at Lankaneu Medical Center says it's likely community-acquired pneumonia. And with her schedule, being around lots of crowds, shaking hands and holding babies, it can be easy to pick up germs.
"Doing what she does for a living, she got the bug somehow and it got into her lungs," Dr. Targan said.
Mrs. Clinton was prescribed anti-biotics and she had said she was taking anti-histamines for allergies, which can make someone more susceptible for dehydration and heat-related illness.
"You have to make sure you're adequately hydrated, drinking enough fluids," Targan said.
Clinton went to Chelsea's home after the incident.
She left about an hour later, stopping for a picture and smiling.
"I'm feeling great," Clinton said.
As for pneumonia, Dr. Targan says recovery takes the right anti-biotics and as much rest as possible.
He says for a 68-year-old woman, such as Secretary Clinton, this is vital.
"Sometimes it takes five to seven days until you fully recover and the older you are, the longer it's going to take you to recover," Targan said. "Most people do recover from a pneumonia such as what she's having, but if she doesn't take the advice of her doctor, she may end up in the hospital and that's definitely not what she wants."
The Clinton campaign has said they've scaled back her schedule this week. As for whether she is contagious, her camp says she is not.
With most cases of pneumonia, people are instructed to wash their hands often and cover their cough.