Is it cheating if my girlfriend messes around with another girl?

I am a guy 22 and my girlfriend is 19. I've been with my girlfriend for 6 months. About 4 months into our relationship, my girlfriend told me that a couple years ago she had a full on lesbian encounter with her good friend. It was at a party but very very late when they were going to bed together and stuff happened. She made it sound to me like they were far from drunk at this point and just really horny. I found this kind of hot actually.

But her and I went to a friend's house of ours to see this friend (a girl) and her boyfriend who were both friends of ours. The girl friend got pretty drunk quickly and started getting pretty physical with my girlfriend. I didn't think too much of it at the time. A little while later, my girlfriend was buzzed but not drunk. My buddy and I were simply playing cards while they went out to smoke and go to the bathroom. Before we knew it they had been gone for about 25-30 minutes altogether.

My buddy and I had to piss so we headed to the bathroom, knocked on the door and was told to go away because they weren't done in there. My buddy walked away but I really had to go so I told them that it doesn't take this long to go to the bathroom. I waited another minute listening and finally decided that I was just going to peek in...as soon as I did my girlfriend ran to the door and slammed it on my face.

My buddy and I both got upset as he came back over and finally told them to get the hell out. They came out looking guilty as can be and I had a bad feeling in my stomach. I asked my girlfriend what happened and she said that our girl friend commented upon her ass and boobs and before she knew it things got physical. They made out and were groping each other.

I felt sick to my stomach and was freaking out. My girlfriend was like it's no big deal...all innocent but what bothered me was that it wasn't like she was trying to turn me on by making moves on this girl to get me going. No they purposely hid in the bathroom and messed around. She did this for her own pleasure not mine or even ours.

My girlfriend said that she's not bisexual but I have trouble believing that after both stories. Also the first story includes a girl who she still hangs out with to this day...they go to the gym together at like 11pm at night and don't show back up for 4 hours usually. It's only once a week usually and my girlfriend still has sex with me at least once a day but I still feel like something is up.

So it is cheating in this scenario right? And do you guys think she is still cheating on me on a regular basis with her friend?