Rapper Mellow Rackz is in police crosshairs after cops say she got into a fight with her boyfriend’s ex-girlfriend and swiped the woman’s cell phone.

Law enforcement sources tell TMZ … Mellow Rackz is a named suspect on a robbery report after getting into a fight in the lobby of a West Hollywood hotel early Monday morning and allegedly walking away with someone else’s cell phone.

We’re told Mellow was in town for Rolling Loud, and her boyfriend booked them a hotel room … and also, unbeknownst to her, booked a room in the same hotel for his ex.

We’re told at some point during the altercation the woman dropped her phone, and Mellow snatched it and left, telling the woman, “What are you going to do about it?!?”

Our sources say police were called to the hotel and officers spoke to the alleged victim … but we’re told Mellow refused to come out of her room to speak with law enforcement, telling them through the door to talk to her lawyer.

We’re told officers took a robbery report and named Mellow Rackz as a suspect.

Our sources say detectives are being assigned to investigate … but no arrests have been made.