Your girlfriend is a flirt. She probably flirted with you before you dated, right? So you knew that this was a personality trait she possessed. You can't control her, even if you are dating her...so you can either sit back and watch or you can break up with her. Either way, be confident in yourself. Just because she is flirting with other guys doesn't mean that there is something wrong with you or with your relationship. She may just like the attention, but that doesn't mean she can't like you at the same time.