I thought I was done with this, but...

Around the beginning of the month, I suddenly developed diarrhea that lasted off and on for days, along with severe cramps, bloating, and gas. Went to three different doctors and was given dicyclomine and promethazine (no idea if I'm spelling those right), to no avail. Finally after a week I became so nauseated and miserable I went to the ER, where they took blood, did some X-rays and a stool sample, and basically told me they didn't know what was wrong with me. Gave me some sort of super-strength anti-diarrhea medicine (I think the MD said it was Immodal or something to that effect?) and some Lortabs for the cramps, recommended I take Immodium, and I cleared right up within a couple days. I chalked it up to it being a freakish stomach virus and went on my way.

Within the past several days, though, I've started feeling ill again -- not *bad* exactly, but just not right. The gas has returned, and I've noticed my stool is at times black and incredibly smelly, and the toilet paper is dark and seemingly streaked with dark blood. But here's the kicker: the MD said my blood tests (I had two done at two different hospitals) both came back fine, and they didn't find any blood in the stool sample they collected. I'm 22 so I doubt it's anything too awfully serious like cancer, and there's no family history of colon cancer or anything like that, so...what the heck is wrong with me? Every doctor I visit seems to think I'm either exaggerating or hallucinating. What's my next course of action?