Blood loss. Blood loss from the gastro intestinal tract is the most common cause. This could be due to gastritis, ulcer, hemorrhoids, diverticuli, or tumor. Nutritional deficiencies can lead to anemia as well. Cancer of the blood is another concern. Seek medical attention promptly.
Many. Need to assess for blood loss from stomach, colon, uterus first. Also pernicious anemia or folate (folic acid) deficiency. Also would check for less common problems such as blood cancers, hemolysis (destruction), underlying chronic infections (e.g. Hepatitis), medication reactions.