Diddy's just been hit with a slew of new heinous allegations ... from an anonymous accuser who says she was still in high school when he, and others, sex trafficked her and gang raped her.

The lawsuit, filed by Jane Doe in federal court, alleges she met Bad Boy Entertainment President Harve Pierre in 2003 in the Detroit area, where Pierre allegedly called her "hot," dropped Diddy's name and invited her to jump on a private jet to go meet the mogul. The accuser says she was a 17-year-old, still in the 11th grade, when this meeting happened.

She says she accepted the invitation and ended up in NYC, where she met Diddy in a studio -- there was a recording session going on -- and she claims he invited her to sit in his lap ... and then they began plying her with drugs and alcohol.

The accuser included photos of that studio meeting in the lawsuit ... and you do see someone sitting on his lap. She's blurred her face in the images.

She says everything's a blur after that, but she remembers Diddy taking her into a bathroom where he penetrated her from behind as she was bent over a sink. She says she didn't consent to having sex. The accuser also claims Pierre and another man who she doesn't name ... also had forced sex with her during this night in the studio.

She claims she ended up in the fetal position on the bathroom floor -- and was eventually escorted out of the studio and flown back to Michigan.

This accuser says she suffered silently for 20 years, and although she knew speaking out would be difficult ... but felt confidence after seeing Cassie and several other women share similar allegations against Diddy.

The suit alleges Diddy violated NY State's Victims of Gender-Motivated-Violence Protection Law ... which has expanded the statute of limitations.

Diddy reacted swiftly to the filing, telling TMZ ... "ENOUGH IS ENOUGH. For the last couple of weeks, I have sat silently and watched people try to assassinate my character, destroy my reputation and my legacy.

Sickening allegations have been made against me by individuals looking for a quick payday. Let me be absolutely clear: I did not do any of the awful things being alleged. I will fight for my name, my family and for the truth."