There are a number of reasons that would make a person avoid you. You may have done something that annoyed your friend and that is why she is avoiding you. You need to talk to your friend to establish her behavior.
There is no one reason. Try asking your friend directly or get another friend to intervene. Avoid sounding defensive or annoyed - your friend might be preoccupied with something else and not even realise how his/her behaviour is affecting you.