Your body uses the evaporation of your sweat to keep cool. That’s less efficient in very hot, humid weather and when you exercise. When your build-in cooling system isn’t effective enough, three things can happen: heat cramps, heat exhaustion, heat stroke.
Heat cramps are the least serious of the three. The symptoms of heat cramps include sweating heavily, feeling tired, being thirsty, and, of course, getting muscle cramps. It you deal with heat cramps promptly, they probably won’t worsen into heat exhaustion.
Treatment of heat cramps is simple. Drink fluids (beer does not count) or sports drinks like Gatorade that have electrolytes. Get into cooler temperatures. If you can’t get into an air-conditioned building, at least get into shade. Rest until you feel well again.