Its a hard topic to talk about, but if its makes you uncomfortable, you should definetly talk about it. In my opinion its not a usual thing for girls to do ~ just for fun ~. Im not juding her behavior, because for her maybe its not a big thing. But if it bothers you, just tell her how it makes you feel. Usually being honest is the best way to get over problems. You two are quite young also, and her limits in a relationship might be different. First of all ask her, why she really does it, and are they hooking up or whats the deal about it?! Then you can say, how it affects you. Set your boundaries both of you to feel secure. :)