Clinton, who was forced to cancel a California fundraising trip on Monday and Tuesday, also faces questions over why her campaign took two days to reveal the pneumonia diagnosis.
A campaign spokesman announced the trip was canceled just hours after the 68-year-old Democratic presidential candidate abruptly left the Ground Zero memorial in New York on Sunday, suffering from dehydration.
Clinton spent 90 minutes at the ceremony greeting relatives of those killed in the terror strikes 15 years ago, her campaign said. The former secretary of state was a U.S. senator for New York at the time of the attacks.
During the ceremony Clinton “felt overheated” so she left for her daughter Chelsea’s apartment “and is feeling much better,” the statement said.
The incident, in which a wobbly Clinton appeared to lose her footing as she was helped into her vehicle, was caught on amateur video posted on social media.
The campaign later released a statement from her personal doctor, Lisa Bardack, revealing that Clinton had been diagnosed with pneumonia on Friday and was suffering from dehydration.
“She was put on antibiotics, and advised to rest and modify her schedule. While at this morning’s event, she became overheated and dehydrated,” Bardack said.
“I have just examined her and she is now re-hydrated and recovering nicely.”
The video posted on Twitter showed Clinton appearing unsteady as she waited to enter a vehicle to leave the 9/11 service.
She appeared to stumble as she was helped to the vehicle, and had to be held up on either side by members of her entourage.