Depends on what you are calling a bloody bowel movement. A black, tarrry bowel movement would be something in the upper gastrointestinal tract, like an ulcer. A large amount of bright red blood is a symptom of a lower gastrointestinal bleed such as a perforation or ruptured polyp. Small amount of bright red blood could be a ruptured hemorrhoid
You could have hemorrhoids, an infection that is seriously irritating your bowels, or a growth or tumor that is causing a problem. If this situation has persisted for very long I would give serious thought to going to a doctor.
The cause for ones bloody movements may be one strains too much in the toilet. In such case avoid taking baked items like brd and biscuits. Soak black rasins in hot water for a night and in the morning squeeze it and drink the water for a few days if no relief consult a doctor.
Bloody bowel movements could be caused by something as minor as hemroids or could be something much more serious such as colon cancer. My advice to you would be to make a Dr. Appointment as soon as possiable.