I had this issue about a month ago. For me it was overtraining for too many weeks in a row. I took a few days off, and when I got back on the bike, I wasn't dead after a ride. Before that - I would be useless the rest of the day. I would eat and then crash for several hours, and I do NOT have time for that! A day in between to rest was not enough (then). Right now it is fine, but I've also leveled off my miles and am in better shape than I was.