Saturday night i began to feel stomach pain with diareah. i drank pepto bismol (bismuth subsalicylate) once that night and since then i have black stool. it is now tuesday.

May be Pepto, but...: Dark-colored stools may result from iron supplements, 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. Stop any nsaid's you're taking, & ask your doctor to rule out occult blood