What Causes Red Blood And Mucus In Stool During Menstrual Cycle?

I'm 34 yrs old, 5 ft 2, 135 lbs, very fit and active person. No medical concerns or conditions, excellent health. For the last couple months I have noticed bright red blood and mucus in my stool but only right before or during my menstrual cycle. I am certain it is not menstrual blood mixed in. I just had a gyno check and pap smear that was normal a few months ago. They have always been normal my entire life. My gynocologist said I had no sign of hemmorhoids. What could this be? Do I need to seek medical attention?