The presence of blood in her stool does not affect blood volume, but leads to iron deficiency anemia when iron stores are depleted.
Iron deficiency anemia is a common type of anemia, a condition in which the blood does not have enough healthy red blood cells. It is due to a lack of iron, so the body cannot produce a sufficient amount of hemoglobin, a substance present in red blood cells that allows them to transport oxygen. As a consequence, iron deficiency anemia can make you feel tired and short of breath.