Bad lower abdominal pain

For nearly three months now i have been having pain in my lower abdomen. Not always there, but recently it has become more frequent and more painful. About 80% of the time I have diarrhea. Usually foul, sometimes black, more recently yellow. Sometimes the pain gets realy intense and radiates to my back. It usualy isnt centralized, sometimes centralized left, or lower center.

I went to doctor, tests were done, Bloodwork found nothing unusual. One liver enzyme was 2 above the average range, but I was told that is not serious. I had an ultrasound, nothing looked abnormal. Occult Blood tests came back negative.