Time of the month "cleansing"

My cousin and I both get diarrhea (or really loose poop) when we get our periods. Is that common? It's not bad or dehydrating, nothing serious - it's kinda nice, in fact, but it's just different than the usual firm. I also wonder why. Know anything about that? (Not related to a certain bowel cleansing but it's like a natural one every month). Tweet