That's one of those things that usually is something non-life-threatening, but it COULD be life-threatening if not diagnosed. Please go ASAP to get it checked! More than likely, it's an ulcer. It could also be an accidental swallowing of poison, or of course a sharp object. I swallowed a piece of sharp chicken bone once, which caused minor bleeding on the way in AND on the way out . Even in my case, they kept checking me for blood poisoning since the bone splinter could have pierced the colon, causing peritonitis (big, potentially fatal infection of the abdominal cavity).