Feeling ill everyday?

Okay so it all started one Pe lesson, we were doing athletics (800m then a 3x200m relay) and I am quite sporty, Im in an athletics club but I prefer sprints. I usually can come in the top 5 at school when doing long distance, but I came 12th and I couldn't run in my lane and I was all dizzy. Our school is on a hill and the running track is at the bottom and I could hardly walk to the changing rooms. In the changing rooms I was very badly sick and overheated. I got sent home. But ever since then, when I eat something or drink something I feel sick, I get full easier, I keep getting dioreah (Can't spell it) when I go to the gym, I can usually go for about half an hour - 45 mins without stopping but now I can only manage 10. I don't know what's wrong. I get annoyed/depressed easier and really bad headaches and migraines :( somone please help me