My girlfriend (28) told me before we were dating she had a sexual encounter with a woman (24).

My girlfriend told me that her and this girl were at a party, they were glancing at each other and giving each other looks. The girl placed her hand on my girlfriend's thigh, they went to the garage where they proceeded to make out. The girl went down on my girlfriend and then my girlfriend did back until the girls roommate caught them and my girlfriend got spooked and left.

I believed this story but I wasn't sure if she was just using it as a fetish to turn me on or what was happening?

Recently, I got the chance to speak to the girl who my girlfriend claimed to have the encounter with (they don't talk anymore and my girlfriend doesn't know we talk). I didn't bring any of it up but the girl mentioned to me that she had a regretful sexual encounter with my girlfriend where they were at a party the girl was really drunk my girlfriend brought her back into her room and went down on her. My girlfriend begged the girl to go down on her back but the girl refused because she said she was uncomfortable with it and didn't think she was sexually attracted to woman. They spent the night together and when the girl woke up out of embarrassment and regret she threw my girlfriend out.

Now I'm not sure whose story to believe. I want to trust and believe my girlfriend, but I feel like either she could have made up the story to further a sexual fantasy or fetish to me. Or there's a possibility she's beyond embarrassed and refuses to speak about it the right way and makes I different in her own head.

What do I do? Who do I trust? I'm not even sure if I should bring this up to her or just leave it be?

I appreciate the advice

Edit: they were both drunk and the girl told me she didn't feel taken advantage of in anyway. She just felt a lot of regret.