Didn't you psychologist go over with you how to answer questions from people at school? That's the first person that should be counseling you.
Otherwise, I might tell people, "Man, my guts. They, like, pretty much shut down. I couldn't hardly eat anything. It was real serious; I was in and out of doctor's offices for months. I was afraid I had cancer, but it's not quite that bad. I'm on medicine to make everything work now, but it's still kind of touchy. I have to be careful what I eat, you know?"