I have the same problem also but what's bugging me is the foul rotten taste I have in my mouth is it from the black stool?
funny thing, i actually have almost the same stats as you-20y/o female doesn't smoke, same symptoms, etc.... anyway, it's from the pepto bismol you guys. i was going to go into the ER. then, i remembered something about this before and looked it up online. i couldn't find anything, so i called pepto themselves & they told me it can turn your saliva and stools black. it's totally normal and there is no reason to see a doctor!
I would consider digested blood from stomach bleeding. It could be from an ulcer of something more serious. Go to the doctor right away.