Rachel Kennedy says it all went down in Japan in 2000, when she and 2 friends met Diddy at a topless club and he invited them back to his hotel suite.

Kennedy, who appeared on the Daily Mail's "The Trial of Diddy" podcast, says she thought she was going to a party -- but when she arrived the only person in the room was Diddy in a bathrobe.

She says she and her friends did coke but Diddy did not. She and one friend went into a bedroom with him, he got naked, and they gave Diddy consensual oral sex for around 20 minutes.

When they walked into the hotel room, there was a music video playing on the TV ... of J Lo singing. She didn't recall the song, but says the video was playing on a loop. BTW ... this was during the same period when Diddy was dating J Lo. The former model claims he was on the phone with Lopez at the time.

After 20 minutes into the sexual encounter, Diddy's bodyguard came into the room and became enraged ... he recognized Kennedy as the woman he was with the night before. She says the bodyguard got physical with the women, but Kennedy says Diddy did not get involved.

TMZ has 2 documentaries on "The Downfall of Diddy" which are both available for free on Tubi. The third doc drops November 12.