Got a girl situation, need advice?

I met this girl in February when I had a little party at my apartment and we seemed to be cool and all, she had a boyfriend at the time. Well fast forward that to about late March when she broke up with him for cheating. I once again came across her at a party in April and we began talking and so forth and sure enough I had a one nighter with said girl, we get along together but at the time I didn't think anything of it. Well about a week later apparently she starts going out with another guy, someone I know who can be a head at times. Shes extremely akward around me when he is there but when he's not we talk, the guy seems like a controlling type, I'm just guessing. Anyways long story short should I just forget about her and the situation even though it may seem like I was the rebound guy or whatever.