right then....well it's a bit complicated and as such you can't really say if your girl friend is right or wrong; but I can tell you one thing for sure, according to my personal opinion the way you have acted is inappropriate. You shouldn't be insulting some one you claim to love, for that matter anyone as you have insulted this girl
I agree; seeing your girlfriend flirting with other guys hurts, it hurts your feelings and it hurts your pride as a man, especially when they say they love you and they wouldn't do that again, you're still 19 and I assume she's a 19 or maybe even younger so it's only a natural thing for a girl to flirt with guys, if you think about it, you must be flirting with girls as well, both of you're still young, these thing tend to happen, if I were you I would personally apologise to her, without calling her a ***** tell her how you feel, and give her the option of either being with you or not being with you