Lower left abdominal pain, pale stool, light-headedness...?

The pain is mostly located in the lower left abdominal region. It radiates from front to back (usually in the front, though) and sometimes moves up under the ribs and to the lower right as well. Usually it is an ache spread over a region, but sometimes there's a sharp, localized ache in the lower left. My bowel movements went from normal to dark (almost black) & mucosy diarrhea, to hard, pale stools accompanied with constipation. This has happened over the course of about a week. I also feel slightly dizzy, nauseous and light-headed. Any ideas what might be going on?