If you're going to do ballet, you'll be exercising more any you have to be careful that you increase your intake, even if that means taking it up to 2000 cal. I wouldn't worry about the amount you're taking in. I'm almost exactly the same as you, 5foot 4 1/2 inches and weigh around 46/7kg (103lb) and want to do dance full time when I leave school so do loads of ballet and dance now. I'm trying to put on weight now because I've got 2ndary ammenorea but again finding it hard, but I'm trying, it's a change of mentality that you've got to get around, you've got to have a mantra in your head saying that you've got to eat, probably the opposite to the one you've got now. Your weight seems quite low, maybe you should think about putting on weight to get healthy, I know it's more normal to be around that weight especially if you're going in to the ballet career, they'll probably be a lot of pressure about that, but you've got to put yourself and your health first. You've got to make sure that you get all the required areas of food and that includes fats etc, and you've got to make sure that you get enough of them. The first thing I would say to do though, is to go and see a GP as the others have suggested, even if you don't have amennorea or any other clear symptoms like that, you can get their advice and see what they say. They'll be able to suggest what to do better than any of us can.