Help does flirty mean slutty?

My girlfriend has surprised me alot she seems to flirt with every guy she knows and the worst part she does it in front of me like im not even there I pretend not to notice but it really bothers me. I imagine how she is when I am not with her or when she is at school(we go to different high schools)so I want to know could she possibly be promiscuous or slutty because all this flirting make me think so.