What should I do about this?

At school, there is this girl in my group of friends and she calls me 'Shagatha' (Making fun of my name, which is Tabitha). I keep asking her to stop but she just carries on. She is on facebook right now, should I say something to her? And what should I say? I'm a really shy person, and she is one of those people who always seem like they are drunk and really hyper, so it's really awkward when it is just me and her talking. How do I get her to stop calling me this? This really annoys me