Medical causes of fatigue. Some common causes of fatigue include: Anemia — When you have low red blood cell counts in your bloodstream, your body has to work harder to deliver oxygen to your cells, causing fatigue. Iron deficiency is also associated with fatigue. If you're diagnosed with anemia, then it's important to understand the cause in order to correct the problem. Bleeding from the colon or stomach are frequent causes of anemia in older adults. It would be prudent to investigate whether colon cancer at an early treatable stage could be the cause of new onset anemia and be cured. Monthly blood loss in menstruating women and frequent blood donations are other causes of anemia.