Replying to Am I overly sensitive?

Alright so first the backstory. I like this girl she lead me on the dated my bestfriend and fucked me over. They broke up awhile ago i'm not friends with my buddy anymore but me and her talk now. I'm taking her to the dance at my school this up coming weekend and was suprised she said yes. Well tonight we were hanging and she wanted to stop off at the guys house because she's good friends with his parents so we go there. While were there we start to watch a movie. She ditches me and goes and cuddles my former bestfriend who also has another girlfriend now. Am I in the right to be pissed off or not?