Painful Bowels

What could be the cause of black bowel movements, from diarea (diarrhea) to
constipation, acompanied by lower back pain, and constant lower
stomach pain that lasts about 2 weeks then things go back to normal.

[HFHSm.D.-Gastro-rf]

Dear NAD,
Black bowel movements, especially if tarry, are often a sign of internal bleeding i.e. bleeding from the stomach or duodenum, usually due to ulcer disease.  Over the counter medications such as Pepto Bismol can also cause black stool.

Given your symptoms of pain, alternating bowel movements and black stool, I think that a visit to your doctor would be a good idea.  If you fee dizzy, weak or light headed (symptoms of bleeding), you should go to a hospital  emergency room