I hate that same old bs response of just walking away without questioning anything. That basically means you are refusing to take any responsibility for any wrongdoing you may have committed. Why not ASK HER what her deal is? Talk to her friend confrontationally. Like 'hey what's your deal? Why do you hate me so much? I'm not intentionally trying to do anything wrong. Maybe if you could tell me wtf I'm doing wrong, I can fix myself instead of hurting [your girlfriend] or any other women in the future.' And you do this in front of your girlfriend so that if she has nothing to say, then your girlfriend will know that her friend is just a bad influence on the relationship. If she actually does have something to say about your relationship, then you Plus, the fact that she called you up crying when she was drunk shows that she has some underlying guilt over something. They say when you are drunk, you show your true self, so that's how you know what her honest feelings are towards you. The fact that you are making her feel guilty over something shows that you are having an extreme emotional impact over her. Why is she having this emotional impact? Because she loves you, a lot.
Next time you see your girlfriend, ask her what's wrong and be genuine about it. Say something like 'hey [gf name], what's wrong? You seemed really upset last time we talked and didn't seem to be having much fun on a trip that definitely should have been', or something to that effect. Her friend will probably be there getting all pissy pissy and that's when you confront that 'friend' on what her deal is. Like I said before, if she doesn't have anything to say, then your girlfriend will know that something doesn't quite seem right. Then you say 'you don't even have anything to say. Are you getting jealous of your best friend or something, or what? Why are you trying to sabotage our relationship when we were happy before?' And she will say 'but your girlfriend isn't happy though' which is where you confront your girfriend asking why she isn't and what happened. It is at this point that you now directly confront the issue at hand and work out possible solutions to whatever the problem is. Walking away isn't always the answer to everything.
Now if her friend actually does have legitimate things to hold against you for, then you either explain how you aren't intentionally trying to cause harm and that you will try to work on those things. Or you can explain why you do those things if they are bullsh!t reasons. Then you ask your girlfriend what's up afterwards.
Now if your girlfriend says that nothing is wrong, you look her in the eyes and tell her something along the lines of '[name], look. I know you well enough at this point to know that something is bothering you. You can't just hold it in like that though, it'll eat you up inside. You need to tell me what's going on so we can at least try to work it out.' If she continues to deny anything, then you ask her friend what's up. If her friend doesn't want to say anything, then you tell them both, 'look, I know something is up that you guys aren't telling me and it's not right that you're both leaving me in the dark. I deserve better than to be treated this way'. It's only at this that you turn around and walk away without looking back, unless they tell you. Then you go NC unless she's initiates.
Your girlfriend is very clearly falling for you, HARD. She opened up a little when telling you about those two guys she had sex with because she was indirectly asking for your permission if it was alright to still be friends with them, which imo is a good sign because it shows she is submitting to you, and trusts you a lot. Plus she called you up on her trip crying, meaning that she was thinking about you intensely. Don't just walk away without warning, it'll crush her and make her extremely jaded towards men, and seek out to crush all of them too. Actually TRY to find a solution by addressing the problem at hand.
