Why is it that every month around that time I have severe pains with bowel movements and when I sit down?

Pain bowel movement. The first thing I would ask is "are you female? ": women can have ovulation pain or even mild endometrial pain with a bowel movement. If you are male, I would bring it up with your doctor so the can do an appropriate exam or study on you. Either way, if it consistently happens, make sure you don't ignore it, and bring it up with your doctor.
Variable. Many women will get more constipated during or just after their menses. Often this is due to poor appetitie and excessive bleeding (fluid losses) and not drinking enough fluids. Harder stool in the rectum along with anatomical changes in the pelvis can cause pelvic pain. Next cycle try anticipating and drink 20 30 more ounces of fluids in the days leading up to and during your cycle.