Accidentally took 36 mg. of iron 3 days ago. after 2 days mild upset stomach/nausea, i am better, but had dark stools this am. is this concerning?

NOT good if melena: Dark-colored stools may result from the iron supplements you took, but also from, foods you eat (like a lot of spinach), substances (like peptobismol), but most significantly suggests bleeding. When blood is denatured by acid in your stomach, as from ulcers, stools will turn tarry ("melena"). Blood may also be ingested from nose bleeds, bleeding gums. Avoid NSAID's & get checked please.