What are possible causes of gastrointestinal bleeding?

MANY. There are many causes for a gastrointestinal bleed. To name a few: ulcers, varices, cancer, medications, arterial venous malformations, polyps and hemorrhoids. The best thing is to see a gastroenterologist who can perform and upper endoscopy and/or colonoscopy to find the cause and provide the best treatment.
Many. Stomach or intestinal ulcers, tumors, and vascular malformations are common causes of black stools (old blood). Colon cancer, diverticulosis, vascular malformations, colon ulcers, hemorrhoids and polyps are common causes of bright red blood in stool. All should be evaluated.