I wouldn’t say you are overreacting. I would be really hurt to know that her friends are bringing their partners along too. Totally understand if it was an all-girls event. I get you don’t want to ruin her birthday so I agree and that you should wait till afterwards. It’s something that she needs to know how hurt you are. One birthday something sour happened and my husband felt bad bringing it up and potentially ruining it. We talked about it afterwards and sorted it out like adults and overall I’m glad he brought it up. If your girlfriend really cares about your feelings she will hopefully understand.