Getting dehydrated + the temperature of your body raises a couple of degrees.
If it happens, lay down flat on your back and raise your feet a little. Drink water with sugar in it- VERY SLOWLY. My mum used to give it to us a tsp every 10 or so mins. Just go slow, or you'll throw it up.
Heat stroke is a condition caused by prolonged exposure to heat. It can lead to dehydration, dizziness, disorientation, and fatigue among other things. It can be a very serious condition if not treated quickly. A person experiencing heat stroke should get into a cooler area right away and drink plenty of fluids.