I always get ill the same way! whats going on,plzzz answer? 10 points?

Whenever I feel I'm getting ill my eyesight goes blurry and then like 10 mins later I get a bad headache and feel ill, I don't get why? I just played football for a couple of hours, in just a football shirt obviously and it happened at the end of it, the weather was nice and cool so I don't get how I became ill.