Hmm, got it. Well, actually I left the party and then my friend called me; it wasn't planned for me to go meet her until I already arrived home.
When I talked about this with my gf, she urged me to put in some research in myself - which I hereby do :) I understand that she doesn't have to explain everything to me. But it also really hurts me when she told me that her two closest friends advised to break up with me if something like that happens again. So it definitely is societally frowned upon.
And thank you for the heads-up. The friend in question gave me a neck kiss, which I talked about and told her it wasn't okay. She is now in a relationship, and I really have the feeling she pays more attention to my boundaries now. But I'll keep in mind to stay vigilant. I don't think she knew my partner had her birthday today, but thank you for the warning regardless - I'll keep it in mind.