I always have diahrrea a couple of days before and after my period. is this common?

Yes: Cells in the uterus produce a chemical called prostaglandins. Prostaglandins stimulate smooth muscles in both the uterus ; in some women they also stimulate smooth muscle of the bowel. As the smooth muscles are stimulated in the uterus it allows the uterus to expel excess inner lining tissue. As smooth muscle in the intenstines are stimulated it propels feces rapidly through the colon. Because of.