Please help am I going the right way with this?

I am a 13 year old girl and am 5ft 5" and weigh 7 st. I have checked online sites and I am not underweight. However, I have recently been obsessed with eating and for a long time have skipped school lunches. My periods have stopped and I am now a peerian. People have rung up my mum and said I am not healthy and underweight. Today, I went to the doctor. She measured my height and weight, but couldn't find the bmi calculator and said that she would ume I was underweight. The past few days my eating patterns have changed to a pigs. Bowl of ceral in the morning with toast and a egg Mayo sandwhich and cereal bar for lynch, then for dinner a jacket potato with basil, spinach rocket and spring onion salad with corn on cob and er. Yogurt for pud and tuna pate and er sandwich on seedy bread a dark chocolate dipped Ginger biscuit and peanut er on bread( seeded). I don't find eating pleasurable anymore it just feels like a task and I think **** it I might aswell stuff my face with crap and I do excactly that. I know I have to but I don't want to put on weight but I'm not allowed to exercise. I used to weigh 8 st 3 lb and I never want to end up like that again. What should I do? I'm turning slowly into a fat person I think help? I just want to be happy I still feel huge around all the other girls in my school but then everyone is underweight? How can I put on weight without becoming obese? Help? Thank you x