Does this make me a jerk?

Hmm, I sort of had this situation (as the girl in the story). But in my case, I had a boyfriend that I didn't really feel I liked - we just started dating because friends hooked us up. Maybe she likes you, but has a tough time breaking up with the other guy, with all these people counting on them to be together. That may be a side of it. I say, talk to her and be forward. There's no need to feel she's playing guys if it might turn out otherwise.

Your reaction is pretty normal. She probably wants to keep being friends and that's why she invited you to the party, but you don't have to go if you would feel uncomfortable there. You didn't know she had a boyfriend when she first asked you, so it's normal that you might want to change your mind about hanging out now that you know.

I think maybe if she had a boyfriend then she should of been upfront with you from the begining. it seems to me that she is just looking at you like a friend and she thinks that you should be ok with it. But I think you see that she was really not that into you like that. I see why you lost intrest. you did the right thing. No it does not make you a jerk

Hmm... are you sure you were actually dating? If so, maybe she was getting ready to branch swing and dump the ex in favor of you? Hard to tell without more info.

Just go cold for a while, don't tell her all logically what you're feeling or any of that. Just stop talking to her for a week or two and see if she starts blowing up your phone wondering what happened to you. When you're ready to hang out just tell her you've been busy and you guys should hang out again and make plans to do something. When you see each other again after the break you'll be able to get a much better read on what she's up to and if she's just playing games or wants to actually date you.