Some antacids like Pepto Bismal can turn your stools dark. There is a real difference between a dark stool and one from internal bleeding. If you are bleeding internally, your stool will look like "tar" and often have blood on it. If you have any doubt, don't flush and call your doctor who most likely will have you take a small sample to the lab. A thoroughly washed pill bottle about 1/4 full is more than enough. Also check your urine. Really dark urine can be an indication of blood, but also be aware that many foods and other medications can affect how your stool and urine look. This needs to be discussed with your doctor as soon as possible.