I just stopped eating? please help!?

Okay, I'm 12 and really want to lose weight so I've gone on a diet. But one day into my diet, I just don't want to eat anything anymore. Even if my mum cooks some food, I'll let her see me take stuff from the pan and when she goes to her bedroom to eat, I'll just put everything back. I don't know what's wrong with me. I won't even allow myself to eat a g because I'm scared it will make me more fat. Do I have anorexia nervosa?