It's called anorexia. However, such behaviour can occur due to an other illness, so I'd recommend a doctor's appointment. But before that, it's important to reflect on the situation and think about the possible reasons for this. Are you satisfied with your appearance? Have you been bullied cause of your looks? If the answers are yes, ask yourself why. Physical appearance is very superficial and shouldn't have such a big role in our lives. Besides, I am sure that you're beautiful and I hope you'll soon be able to realise that too. The recovery starts from within, from self love.