Hello from JustAnswer.
A bowel movement that is completely dark red is typically seen when there is a large amount of bleeding into the gut either high in the colon or the lower small intestine. Bleeding into the lower colon is typically bright red, and bleeding into the stomach or upper small intestine is typically black.
There are many possible causes of bleeding into the upper colon or lower small intestine. Cancer is certainly one of the primary concerns, but significant bleeding also can occur from benign growths, vascular abnormalities, or significant inflammation of the colon. The usual test to determine the cause of the bleeding would be a colonoscopy.
If you are also having symptoms of significant blood volume loss, such as lightheadedness, dizziness, a rapid heart rate, or a drop in blood pressure, then it would be important to be seen emergently. Regardless of the cause of the bleeding, a large amount of bleeding should be address emergently.
If you are not having these symptoms, then it is something that does not need emergency care, but it would be appropriate to be seen by your doctor as quickly as possible.
If I can provide any additional information, please let me know.