A councellor at school is a great idea, they've probably dealt with issues like this before so they might be able to give you ideas on why this is happening and how you can feel better.
What if you joined another team, say like volleyball or soccer, or even did yoga or something like that? This way you'll feel better about eating the things that you want because you are still active. I know already that active people eat more in general because they burn more, maybe because you have stopped your basketball, you have just felt less hungry naturally. But still the obsessing thing is probably not good, and it would be great to talk to someone. And when you don't feel like eating, perhaps you should just make yourself anwyay, even if its something small like an apple.
But please pleeeease do not throw up - i have been bulimic for a year now and have a lot of problems going on, you think you will stop after one try but its never the case. so please don't start that.
good luck and stay positive!