It's hard to describe it. I don't feel ordinarily bad, other than the obvious that goes with a ride that you've put effort into. My best guess was a shortage of water, since I don't have the capability to carry it and don't keep track of time while I ride. But otherwise, I'm not really sure what would cause that. It doesn't happen any other time, so I'd have to think it's something to do with the physical exertion of the riding.