What Causes Headache After Playing In Sun?

I get a headache at the back of my head after playing in the sun. This doesn't happen on other days. I play cricket and I find that when it is not so sunny and hot, I don't have this problem. But when it is very hot, any exposure over 2 hours seems to be causing it. Could you let me know what the reasons could be and what I have to do to minimize it/cure it?