Menstruation periods come due to the ovulation cycles in women. These in turn are caused by the hormone estrogen. During this time, women's physiological makeup react in different ways - some with abdominal cramps and diarrhea, others with headache and emotional upset. If directly caused by the menstrual period it will be transient. However, diarrhea if severe and more than a day can be a serious concern. If oral rehydration does not suffice, consulting a doctor immediately is the better option.
Diarrhea during menstruation is believed to be caused by release of a hormone called prostaglandin during your period. Prostaglandin are the hormones that regulate smooth muscle contraction, this involves the muscle that is found in your intestines. Their main purpose during your period is to contract the uterine muscle in order for blood to be released from the body. But since it also acts in the intestines, diarrhea also take place.