Black stool and abdominal pain for a few days now. could it be symptomatic of stomach cancer, colon cancer, ulcer? i am a worried

Probable ulcer: Black stool suggests blood arising from region of stomach where the effect of stomach acid degrades the hemoglobin to appear black when reaching stool in rectum. The associated pain in abdomen goes along with an ulcer. If symptoms subside with H2 blocker and antacids, this reinforces stomach as site of bleeding. If needed because of persistence of symptoms, gastroscopy indicated.