That happens to me, oddly enough, when I try to go for an "easy" ride. I've always been very active in sports but fairly low energy when not working out. I find that if I don't prepare myself mentally for exercise and get the adrenaline rush that I end up crashing. If I try to go for a relaxing ride, or just "shoot a few hoops" I end up having the shakes and needing to lie down (or is it lay down?).