Bright blood in stool any time indicates hemorrhoids or problems in the colon. There is no direct relation of menstrual blood and blood in stool. So, it is necessary to investigate the underlying cause of this problem. In my opinion, you have to visit to doctor for the diagnosis of bloody stools and its treatment.
The above answer is BS, just checking on line you will find that there are several correlations between blood in stool and menstrual cycle. Endometriosis, ovarian cysts among other cancer. GO TO THE DOCTOR ASAP!
Blood in stool during menstruation can be an indication of endometriosis. Get thee to a gynecologist.
I am having the same problem, today is the first day of my cycle and I have had blood in my stool today. This has never happened before. I do not have any other symptoms that follow Endometriosis or cysts, I only have the normal cramping with my cycle that I always get. I wonder if it is because we are a mess down that way. Post back and let us know what the doctor has said.