This happens to a LOT of people with running. Pushing too hard seems to be the most obvious issue for you right now, as many have suggested. It's best not to run in hot and humid conditions if avoidable. Go in the AM. Also, try not eating too much 1-2 hours before the run and reduce fiber intake in that time frame, especially if, no, WHEN you go to a road race you want to try hard. I've had running partners that do a "pre-run", run too, sort of try to trick it and get it done.