Another thing to consider is that you might be dehydrated. One thing I do fairly regularly is weigh myself before and after a ride, to remind myself how much water I've sweated away. I can ride for 20 miles, drink 24 oz. during the ride, and end up 2 or 3 pounds lighter when I get home. If you do that day after day and don't rehydrate, you're going to feel fatigued.