The theory behind this is that there may be some common disturbance with smooth muscle, as all these problems involve it. Stress and IBS go hand in hand and women are know to feel more stress. And while IBS doesn't cause pain that awakens, it is associated with sleep disturbances. Many women without IBS report gastro-intestinal complaints around their periods or menopause. For women with IBS, this is much more prominent.
Most studies show more gas and cramping.
But others show more constipation before the period and more loose stools at the start. As mentioned, dysmenorrhea is more common in IBS women. This is associated with the loose stools some women experience with cramping at the onset of menses. The relationship with Chronic Pelvic Pain and IBS is confusing. Some believe the common mechanism of smooth muscle derangement is reponsible.