she's trying to get a rise out of you. You're ignoring her so her being mean is completely pointless now. I bet she's following you around so you'll get irritated and get mad. It's possible that she just gets a kick out of you blowing a fuse.
you can continue ignoring it (will possibly irritate her more), start being really nice (girls who are mean find it really difficult to be so when someone is being nice to them), or just outright ask why she's following you around.
Honestly I'd continue ignoring her or being nice. If it happens again just start of by say "Hi *insertnamehere*, Hows it going?" be chirpy and nice. If she's trying to be mean it'll break her resolve