Anemia occurs when you have too few red blood cells. This can cause weakness, fatigue, shortness of breath, pale skin, and affects the function of other tissues in the body. Anemia is caused by too little production of red blood cells, loss of red blood cells as in menstruation, and inappropriate destruction of red blood cells. It is helpful to determine the cause of anemia to provide the most effective treatment. Iron deficiency is a common cause of anemia, but B12 deficiency and other causes exist. Lab testing is a particularly important step in making that determination so your provider can help you with treatment.