No doubt, you need to see your doctor. Some thing is up.

I sure hope you had this checked out. It's hard to say what it is until you've had an abdominal exam and some lab tests. Has there been any changes in your diet lately? Is your stool the same shape as always been or is it stringy? Are you experiencing any dizziness and lightheadedness?

The change of color may be due to food or a more serious reason such as internal bleeding. Do not ignore and seek your dr's advice. Good luck