Is it normal that someone stops being friends with me?

When I was in seventh grade, I met a girl sitting outside on the curb during gym class. It was free day, so nothing exiting happened. We talked with one another almost every day. We entered highschool together as freshmen and continued to be friends and even had a class or two together. We sat by one another in class quite frequently. Once we became upper classmen things started to change. We started off as friends first semester, but then one seconde semester hit, it was like she did not even know me anymore. The only time I see her is on the bus and all we do is just sit next to one another. It is like she is angry with me for some reason or something. Is this normal?