Friends?

Hey! I was just thinking about this girl who I use to be friends with. We would hang out and we got along really well together. We had the same kind of attitude and we just got each other. It had been a couple months since I saw her and we ran into each other at the theater. She basically acted like we weren't friends. It was just weird because she only acted like she knew me but not that well? I know this is childish and sorry if this annoys you guys. Im just wondering why girls will just act like they don't know you. I'm a girl but I've never understood why other girls do this all the time. How can you be friends and then all of a sudden you aren't anymore?