What causes this intense/sharp pain underneath my left breast?

i am a 17 year old female and for months now i keep getting this intense and sharppain rightunderneath myleft breast. things such as breathing even hurt badly; sometimes it happens after i workout, but sometimes i will be sitting on the couch and it will just randomly hurt. i thought it was from wearing a bra but it will happen without one as well. it feels like it could be muscle but i want others thoughts...