Am I unhealthy?

Every time I eat I get a stomach ache while I'm eating it, so I have to stop eating. Especially sugar. That's not good though since I'm already underweight. My doctor wants me to gain pounds but I can't do that if I take mandatory cardio kickboxing cles and can't eat, but I'm starving! It's been going on for like a month! SOmetimes I can eat though, but i can never eat sweetes anymore which is sad, since I'm still young. (I'm 14, not 9)