Is it normal that my friend is becoming increasingly closed off?

Did you sleep with her boyfriend or something??? Unless you had done something really bad behind her back, I can't see why anyone who you are/were best friends with would act like that. I don't think it's normal, it's just plain rude! If you still want her as a friend I suggest you talk to her calmly and figure out what's up her ass. If she responds with the same bitchy attitude than lose her