I dont know why can't i trust my girlfriend?

I understand where you're coming from completely. Chances are your girlfriend wouldn't like it if you went alone to the movies with another girl even if she was just a friend, but I don't think your girlfriend is looking at it like that. She probably didn't even think about it in that way you know? She just saw it as catching a movie with a friend, which is probably all it was! I guess what I'm trying to say is that it's alright to feel a certain type of way about it because almost everybody would! However, it's not worth stressing over really! Don't be mad at her because you guys DO need time apart to hang out with friends and have space or else your relationship will suffer from constantly being together you know? Just talk to her and let her know why it upset you, but don't do it offensively or let it turn into an argument! Just keep calm and let her know! Tell her that maybe next time she could let you know before going instead of telling you after the fact (I find that's better because if they're letting you know they probably have nothing to hide you know). Anyway I hope this helped!! It'll work out just keep calm and talk instead of getting angry!! =)