I agree with Scott. Something is not right. Do your ferritin and Hgb increase appropriately with infusions and how low does your hgb drop in between? Anemia is generally due to one of three things or a combination: blood loss, RBC destruction (hemolysis in the vessels or outside of them), or poor production. If you have an appropriate increase in RBC mass and hgb with iron followed by a precipitous decline, blood loss seems likely. The life span of an RBC is ~120 days and your body should replace these dying cells if you have enough substrate (iron). You could be hemolyzing as well and have imparted production from chronic illness but these are less likely if you respond well to IV iron. You need a thorough work up for a source of a bleed and if none is found, a hematologist work up (it’s typically not very difficult to characterize anemia).