Talk to her. If she completly blows you off, or gets an attitude on, then she could well be bad news. If she is any kind of decent human, she will understand that what she is doing gets to you (even if it seems quite innocent to her) and she should stop.
But for all you know, this is just harmless messing around. You need to ask her, try and get into her head. But be ready to suss out excuses and lies.
Above all else, don't get angry and don't accuse her. She is more likely to get defensive and full of attitude if you do. Be nice, be calm and just ask her.