Dehydration was my first thought too. One way to check is to weigh yourself before and after your ride. In hot weather, even though I drink during my ride, I can lose two pounds and sometimes as much as four pounds in a 20 mile ride. Water weighs eight pounds a gallon, so that's a net loss of a quart to half a gallon of water. I don't seem to be able to drink enough as I ride to make up for the water I'm sweating out. That weight loss tells me I need to be drinking a lot after my ride to rehydrate. I've felt that fatigue too, but it wasn't rest I needed, but water.