Clinton was being treated with antibiotics and had been "advised to rest and modify her schedule," the doctor said.
Reiterating the campaign's earlier statement, the doctor said that Clinton left the 9/11 memorial event because, "she became overheated and dehydrated."
"She is now re-hydrated and recovering nicely," the Bardack said, saying that she had recently examined her.
The Clinton campaign has dismissed these claims as "deranged conspiracy theories."
After the Democratic candidate departed the 9/11 memorial event on Sunday morning, her campaign released a statement saying that she attended the ceremony, "for just an hour and thirty minutes this morning to pay her respects and greet some of the families of the fallen. During the ceremony, she felt overheated so departed to go to her daughter's apartment, and is feeling much better."
According to the National Weather Service, around the time that Clinton was attending the event, conditions in New York City were partly cloudy with an air temperature of 79 degrees Fahrenheit and a heat index of 82 degrees.
The campaign has not commented specifically on a video taken by an audience member at the ceremony that appears to show Clinton being assisted as she got into her campaign's van.
She waved at the press and walked to her van. Asked again by a reporter if she was feeling better, Clinton said: "Yes, thank you very much."
She returned to her home in Chappaqua, where she remained for the rest of the day.
Late Sunday evening, Clinton spokesman Nick Merrill said she would not be traveling to California on Monday or Tuesday for campaign events that she had scheduled there.
Clinton's Known Health History
Clinton, like Trump, has not released her medical records in full detail.
Last July, her campaign released a letter from Bardack, summarizing her current health status, and noting that she had hypothyroidism and suffered from seasonal pollen allergies.
She also suffered from an elbow fracture in 2009.
"She is excellent physical condition and fit to serve as President of the United States," Bardack concluded in the statement.
In 2012, when she was secretary of state, Clinton "while suffering from a stomach virus ... became dehydrated and fainted, sustaining a concussion," according to Philippe Reines, a state department spokesman, in a statement released at the time. Follow-up exams found a blood clot, forcing her to seek treatment in a New York hospital.
Bardack's letter released by the campaign last summer noted that as a result of the concussion she suffered from double-vision, which was treated with special glasses before it resolved itself "within two months."
The doctor's letter said that the clot, called a transverse sinus venous thrombosis, was treated with anti-coagulants and that follow-up tests conducted in 2013 "revealed complete resolution of the effects of the concussion as well as total dissolution of the thrombosis."
It also stated that Clinton, "tested negative for all clotting disorders," but that doctors had decided to continue a daily regimen of anti-coagulants as a precaution.
ABC News' Matthew Claiborne, Veronica Stracqualursi and William Gretsky contributed to this report.