The video -- which shows Chyna performing oral sex on a man -- was leaked Monday on Twitter. Her ex-bf, Mechie, claims to be the man in the video.

Sources close to Chyna tell TMZ the scene was shot on her cellphone last July, but insists she showed it to absolutely no one. We're told the phone was not stolen and there's no obvious evidence it was hacked. No photos or videos from her phone have been leaked, other than the sex tape.

Mechie says after he shot the vid on Chyna's phone, he gave it back to her, and never got a copy of the video.

We're told Chyna will file a police report with the LAPD by week's end and will fully cooperate.