5:22 PM PT -- We've learned police have listed Megan as a "victim" in the incident. She was taken to a hospital immediately after Tory was arrested, and treated for the wounds to her foot.
Law enforcement sources also tell us 4 shell casings were recovered from the scene outside the home.
Tory Lanez got busted after an alleged argument outside a house party led to cops finding a gun in his car, and Megan Thee Stallion was there too ... this according to law enforcement.
We've learned police got a call of a disturbance outside a Hollywood Hills residence around 4:30 AM Sunday. When officers arrived, we're told witnesses reported people were arguing in an SUV before someone fired shots in the air ... and the vehicle took off.
Our law enforcement sources say cops got a description of the SUV and located it shortly afterward, and found the rappers inside along with another woman.
We're told Meg had a cut on her foot believed to be from broken glass on the floor of the vehicle, and when cops searched it ... they found a gun.
Our sources say Lanez was taken to jail and booked for carrying a concealed weapon in a vehicle ... a felony.
It's interesting ... earlier on Saturday night, Megan and Tory appeared together on her Instagram Live with Kylie Jenner -- at what appears to be a pool party at Kylie's Holmby Hills pad.
The reported gunshots were allegedly fired several miles away in the Hollywood Hills neighborhood.
We reached out to Tory and Megan ... no word back, so far.