US Democratic presidential candidate Hillary Clinton on Sunday left early from the memorial service marking the 15th anniversary of the September 11 terror attacks after she felt "overheated", according to a campaign spokesman.A video posted on Twitter shows Hillary Clinton stumbled and she was being helped by her staff into a waiting van, Xinhua reported.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 the 68-year-old 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.