TMZ has learned the woman -- a 21-year-old named Haley -- hired an attorney ... who tells us Haley suffered multiple injuries to her neck, jaw and back after the attack.
Her lawyer Tim Holleman also thinks she may have suffered a mild concussion ... since she doesn't remember the punch. Even though our video shows Haley wandering on stage -- Holleman says the venue should bear some responsibility since security did nothing to remove her. Especially since Afroman has a violent history with fans.
Holleman adds ... he's not buying Afroman's claim he didn't know Haley was a woman ... because Afroman saw her when she got on stage -- something the video seems to prove.
Haley's attorney says she's already racked up medical bills, and she'll be seeking reimbursement ... if they go forward with a lawsuit.