It's usual to have more regular (increased calls to the bathroom) bowel movements during menstrual bleeding.
Increased water intake is very beneficial for regulating the bowel movements, too.
Try to drink plenty of water and eat food rich in fiber.
Constipation can be a part of Irritable bowel syndrome, if it is accompanied with abdominal cramping and bloating.
Hormonal imbalance, such as decreased levels of thyroid gland hormones, causes constipation as a symptom, since this condition causes whole metabolism to slow down.
Using some medications, like antidepressants and others, can lead to constipation as a side effect.

Best wishes!
Marija