Today and once before I faced sickness during a match. At first I thought it was just nervousness, but then came head aches, and the feeling of wanting to throw up everytime I ran. This took all my energy away, and eventually I just could not play a complete point. During this time I was drinking lots of water, maybe 3 bottles, and I had eaten a sandwich before the match. I consider myself to be in good shape, and it was weird being tired from the first point to the last, so tired I wanted to fall asleep or pass out. It was 90 out, could this be heat sickness, how do I prevent this??