You can tell her how you feel. A girl shouldn't cheat on you. If she's going to go out with other guys and you sit at home paranoid - obviously there is a trust issue. You need to talk to her and get everything situated and worked out. She seems like a whore, but i'm nto a big fan of calling girls whores or sluts. If she keeps doing this after you confront her, then find someone else who is mature with their body and consentual feelings with their boyfriend.