NEW YORK, Sept. 11 (Xinhua) -- U.S. Democratic presidential nominee Hillary Clinton was diagnosed with pneumonia at home after she fell sick during a memorial service marking the 15th anniversary of the Sept. 11 terror attacks here on Sunday.

U.S. Democratic presidential nominee Hillary Clinton speaks on the last day of the 2016 U.S. Democratic National Convention at Wells Fargo Center, Philadelphia, Pennsylvania, the United States on July 28, 2016. (Xinhua/Li Muzi)

Local media quoted physician Lisa Bardack as saying "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."

A video posted on Twitter shows the 68-year-old Clinton stumbled and she was being helped by her staff into a waiting van and left the memorial service early.

According to her campaign, Clinton had a rest in her daughter's apartment in Manhattan. As she went out the apartment shortly before noon, Clinton said, "I'm feeling great."

Clinton's rival, Republican presidential candidate Donald Trump and his supporters have questioned Clinton's health, saying Clinton is physically unfit for the White House, accusing her of being "exhausted" and sleeping too much.

Last July, Clinton released a letter from her doctor saying she was a healthy female whose health issues were only as bad as hypothyroidism and seasonal allergies.

Doctors also found a blood clot at the time, but the letter says those issues were mostly cleared up after two months of various treatments.