CBC was ok. Day later drew a "full iron panel" (what's that & why check?) & said my iron levels were very low & put me on ferrous sulfate 325 mg.

Iron deficiency. Most of the time iron deficiency is detected when the CBC is abnormal. However, some cases may still show numbers in the normal range, but tending towards a pattern consistent with iron deficiency. If the iron, iron binding and serum ferritin (panel) all point to deficiency then treatment is good. But should also ask why iron is low.