Mucus and blood in stool during my period

I am 39 years old and never had a problem with hemerrhoids or constipation even during my 2 pregnancys.  But now every month during my menstrual cycle my bowel movement become irregular and very painful. I get very gassy and pass bloody mucas and there is blood on my stool. I will feel the urgency to have a bowel movement only to pass bloody mucus and gas. I will get very painful cramps that are relieved after having one of these so called bowel movements but it can be very painful in by colon and rectal area to even pass gas. The pressure is unbearable making it impossible to push out a bowel movement. I have had a colonoscopy about 6-8 months ago which didn't show any abnormalities and this has been going on for over a year. Again, this only a day or two before until a day or two after my period. What could be the problem and why is it related to my period?