Okay well first i'm gonna start off by saying, you're 12. you would be rediculous if you go on a diet, however it's never bad to be aware of what you put on your body and go for the healthier choice! I dont know what 7 stone 4 is so i couldn't tell you but im sure you're not because fat people know when they're fat, they dont have to ask. Another thing, you do NOT want to be super skinny, super skinny is gross looking, just try being healthy without doing extreme things like dieting and losing like 50 pounds which would be horrible and would probably look bad. But to answer your origional question, no it is not bad to diet while on your period. I hope I helped! (:

I've heard about the fact that if you lose weight suddenly it can affect your period...so my advice is to embraca a diet that allows you to have at least 3 meals per day..and drop the idea of embracing a crash diet....lose weight with a few simple tricks instead of starving yourself to death..check out these tricks by BG ..

http://www.fitness-diet.becomegorgeous.com/healthy_diets/6_wise_dieters_weight_loss_tips-7251.html

http://www.fitness-diet.becomegorgeous.com/healthy_diets/no_diet_weight_loss_methods_to_try-4956.html

Okay, really, why do you need to diet? I understand the fact that if you're overweight, it's hard to not wish you could get rid of it. Trust me, I've bee there, and I'm still a little overweight, but all I had to do was start eating healthier, and I dropped what I didn't need. It was slow, but that's okay. But to answer your question, yes, it can. Your metabolism can change if you exercise and dramatically change your diet. The change can actually make you skip periods until your body becomes used to it.