Well, people are right here about going to a physician and having a simple blood test to eliminate obvious problems.
But it could be simple also.
Some people subscribe to the "no pain, no gain" school and think that when they exercise they HAVE to constantly beat up their body. I don't know if you are one of these people or not. The body needs to recover from any hard exertion. So you either have to ride at a moderate pace on some days for "recovery" rides or more drastically take a "rest" day. Now if you are not beating the daylights out of your body by working out too intensely, than you should, by all means go see a physician.