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1 These are common and often don't cause symptoms, but may lead to a small amount of blood in your stool.  sexually transmitted infections (STIs) â any sexual activity involving the anal area can spread STIs, which may sometimes lead to rectal bleeding.
Melena is black, tarry stool and typically indicates upper GI bleeding, but bleeding from a source in the small bowel or right colon may also be the cause. About 100 to 200 mL of blood in the upper GI tract is required to cause melena, which may persist for several days after bleeding has ceased. Black stool that does not contain occult blood may result from ingestion of iron, bismuth, or various foods and should not be mistaken for melena.