I think she's upset over this trivial thing b/c she is expecting you to hurt her back. She did it, so you will do it.
I imagine she is acting irrationally b/c she is very emotional at the moment. Consider the situation of finally owning up to everything, maybe starting back up with the person you love, and they go to a party. Them going to a party without you is scary enough, given that they have every reason to want to hurt you - but then, your exgf is there too.
Your exW hasn't been DB'ing hard for a year, she was probably running from everything for a long time. She has a journey to go on too.
Patience, patience, patience.
H 34 W 29 BD 3/12/18 Divorce Busted Spring 19
It is not things that bother us, but the stories we tell ourselves about things.