Hmmm, I understand. I can add some more information about it - let me know if you think it should be included in the main text. I didn't include it because I didn't felt it necessary for the question, but I'm not sure about it anymore.
Me and my gf got together in August of last year, so basically 8 months ago; we were friends for half a year before that. She got cheated on in her long distance relationship before.
The friend who invited me to the sea I actually know for almost as long as my partner, from the beginning of Uni. She had a breakup from a three-year old relationship a few months ago, and I was there to support her. I didn't clarify before though if she was okay with me cuddling with people or not; I assumed it was with her, because it was okay in her LDR before - which was wrong of me. I overstepped the boundaries of my partner here.
The friend in question kissed me at the neck while I was at hers. I talked with her about it and let her know that I wasn't okay with it, to which she reacted quite hurt. She then told me that we shouldn't be friends, but two weeks ago she collapsed at Uni and I brought her home. Now we are meeting again.
While I'm writing this down, I'm actually starting to notice that there are a lot of other factors playing in why my partner is upset here. She has been cheated on in the past, which definitely leads her to feel uncomfortable about my actions, even though I obviously don't want to cheat. I broke a societally unwritten rule of not meeting people in romantically coded settings on your partners birthday. And I overstepped the boundary of my partner before by cuddling with the friend without my partners consent.