Competition in 3 weeks, can't eat? tired and weak? help!!!?

A while ago, I started becoming full all the time. I never feel like eating, and even if I do eat (even just a few bites), I become full and nauseous. Doctors have no idea what's wrong with me. I haven't eaten more than a few rice krispy treats and juice in about four days. I'm losing weight very fast. The problem is, I have a dance competition in 3 weeks. I haven't kept my strength up or stretched because I'm so tired and malnourished. But I need to compete. I have a solo and two group dances in which I need to be in. So, what do I do if I can't eat? Is there anyone who may know what i have? I need to get it fixed, FAST!