Sounds similar to BPd but that does not mean she has that. Depending on her age I would not even say that. It could be a lot of different things, from being a bully who is insecure and feels better about themselves by putting others down (even people they are sometimes nice to) to some psychological disorder, or a strong fear of abandonment, there really is no way to tell. Don't let her bring you down, there are a lot of bad people in this world and they are not worth being around.

Either way this girl clearly has some self esteem or control problems.