Have you recently made a big change in how much you exercise or what/how much you eat? If not, exercise may not be the problem. Chronic over-exercise, especially if combined with under-eating, can cause fatigue. But if you haven't changed your routine recently, it could be the fatigue has other causes--poor sleep, stress, a medical problem, etc. If it continues, it might be worth mentioning it to your doc.
Hope this helps.