Hillary Clinton: 'Medical Episode' During 9/11 Ceremony

5:15 PM PT We emailed Clinton's rep and asked why the campaign did not disclose the pneumonia when they first issued a statement saying it was dehydration, despite the fact they've known the diagnosis for 2 days. So far, no word back.

3:00 PM PT -- Hillary's doctor released a statement saying she was diagnosed with pneumonia on Friday, was put on antibiotics, and advised to rest and modify her schedule.

The doctor says Sunday's episode was due to overheating and dehydration, but Hillary is now, "re-hydrated and recovering nicely."

Hillary Clinton had to leave the 9/11 memorial ceremony early because she either appeared faint or actually fainted ... and the video is striking.

A law enforcement source tells Fox News, the Democratic Presidential candidate had some sort of "medical episode" and left abruptly in a van. According to Fox, a witness claims Clinton fainted, her knees buckled and she lost a shoe as she was helped in the vehicle.

A spokesperson for the campaign says Clinton "felt overheated," but went to Chelsea's nearby apartment and felt better.

As Hillary was leaving Chelsea's apartment, she posed for a quick photo-op with a little girl, and said she was feeling much better.