Are you a long distance runner or a short distance runner? The type of running that you do is also important. You may have chronic fatigue syndrome
. The most likely cause is that you are over training. You have to understand that resting your body is as important as working out.
Another thing that might be wrong is that you are not getting enough sleep. You might also not be taking sufficient calories and your body might be in negative calories. If you are a long distance runner, you easily need an extra 1000 calories for a 10 to 15 km run. I advise you to download a good book on running to know the appropriate diet for you. Also, If you do not have any chronic back pain
or aching joints, it is unlikely that your condition is serious.