When to see a doctor. Make an appointment with your doctor if you're feeling fatigued for unexplained reasons. Some anemias, such as iron deficiency anemia or vitamin B-12 deficiency, are common. Fatigue has many causes besides anemia, so don't assume that if you're tired you must be anemic. Some people learn that their hemoglobin is low, which indicates anemia, when they go to donate blood. If you're told that you can't donate blood because of low hemoglobin, make an appointment with your doctor.