It follows Mrs Clinton being taken ill in New York today.
Mrs Clinton arrived at the event at Ground Zero where the twin towers fell 15 years ago at 8.18am local time.
But she left around 90 minutes later.
Reporters who attempted to follow her were blocked as her security team helped her leave the event.
Her campaign team later revealed she had been “overheated” but was now feeling “much better”.
Now, video footage of her struggling into her car have emerged online.
Mrs Clinton can be seen leaning against a bollard to keep her upright, but as a car pulls up for her to get in, she struggles to make it two feet to the car.
She is seen stumbling and her knees buckle before members of her security team helped her into the waiting vehicle.
Donald Trump’s rival was taken to her daughter’s nearby apartment to recover.
US journalist Rick Levanthal of Fox News said: “She unexpectedly left early because of what appeared to be a medical episode.
“I have a law enforcement source who was there, who was 15 feet away form Hillary Clinton. He said she was standing on a curb with her protective detail waiting for her motorcade.
“They were surprised to see her because she wasn’t meant to be leaving yet.
“When it finally rolled up my source said she stumbled off the curb, appeared to faint, lost one of her shoes, which wound up under the van.”
Nick Merrill, spokesman for Clinton said: “During the ceremony, she felt overheated so departed to go to her daughter’s apartment, and is feeling much better.”
Mrs Clinton was later seen leaving her daughter’s apartment at about 11.45am, smiling and waving to the scrum of cameras and posing for a picture with a young girl before stepping into a campaign vehicle.
Before heading for her home in Chappaqua, in New York state, she said: “I’m feeling great. “It’s a beautiful day in New York.”
The campaign team did not immediately say if she had seen a doctor.