This happens to me most of the time after running in extreme weather (either hot or cold). I think its a combination of changes in blood pressure, dehydration, electrolyte imbalance, and in yesterday's case: waiting a really long time to eat after the race. Hell, I felt almost this bad afterwards and all I did was stand outside and watch the race. The only thing that usually works for me is sleeping it off and of course food and gatorade.