The woman 50 Cent punched square in the chest is now going full bore against the rapper ... lawyering up for a lawsuit.
Donnetta Derr has a law firm behind her that will sue 50 early next week. She'll also sue The Lox, with whom Fiddy performed, and the venue, Baltimore Soundstage.
TMZ broke the story ... 50 was performing last week when the woman grabbed his arm and pulled him off stage. 50 retaliated with a vicious punch.
Donnetta later went onstage and twerked ... leading people to believe the beef was squashed ... nope.
Attorney Warren Brown tells us Donnetta went to the hospital the next day and hired him Monday.
Brown calls 50's conduct "cowardly."