I've been diagnosed with anorexia

Last week I went to the doctor and after asking me some questions she said that I have anorexia. I was shocked. I've never thought about it cause my weight fluctuates a lot but I've actually never gotten too thin ( 5'1" , 133pounds).But at the same time I realise how obssesive I could be with calories , food, exercise, getting fat,..I've gone through a hard time all those years , binging, fasting, taking laxatives...I'm worried cause I can barely concentrate on my daily activities such as study. My teacher has asked me today to give him a reason about my ups and downs. I'm just ashamed of telling him about this, I really don't know what to do .