(throwaway account)

So this girl and I have been dating for around 11 months. We're super chill and things were going great. 

At the end of the year, things get weird. She went out of town for NYE and told me it's a huge friends gathering. Turns out, it was just her and her most recent ex partying it up.

She told me all this later, like all her friends bailed so it wasn't a big deal but it felt like she was being evasive about it. When her friends ask who she went with, she doesn't tell them. When my friend asked her, she just ignored the question.

AITA for not being happy about this? I told her that I'm not being possessive or anything but I don't think it's cool she went off with an ex. She says I'm overreacting and he's like a brother.