Confused

Oh yea i forgot to add..one time we all went out, a bunch of our friends. When we were on the way home in the car, it was me(bf), my friend and this other friend sitting in the back. We were all squished. Well i was sitting on his lap and she was next to him but her back was kinda to him. Well he was really drunk and he was like scratching my back, when i looked over he was scratching her back! I know this might be stupid of me but i just got so mad. I kept it in of course He had one arm around her and the other scratching her back, he only does that to me. She didn't say anything, i guess she figured oh it's not a big deal, we're all friends. idk. He was really drunk so idk if that explains it but still, we always agree that we know what we're doing when we're drunk and that it's no exception. The next day he felt sick and didn't remember anything at all. I didn't mention it but i was mad and he couldn't tell why i was mad. I was actually mad for a couple of days and kind of ignored him for a while and then i put it to the back of my mind. It just made me think cause again i know it's prob. a stupid reason to be mad but he would never go and just scratch one of his friend's backs, that's so not like him ayayay