I have chronic diarrhea, abdominal pain(losing sleep becuz of it),nausea.sometimes my poo is completely black(very dark).what could i have?

Black stool: Black stool, particularly when it is somewhat loose and foul-smelling, can be a sign of internal bleeding in the digestive tract. If you see black stool, you should have it checked out to see if it is due to bleeding. Otherwise, try increasing your fiber to see if it helps with your loose stools. If it does not help, see your doctor. Good luck.