What causes intense back pain just before menstrual cycle?

See below: Primary dysmenorrhea is another name for common menstrual cramps. Cramps usually begin one to two years after a woman starts getting her period. Pain usually is felt in the lower abdomen or back. They can be mild to severe. Common menstrual cramps often start shortly before or at the onset of the period and continue one to three days. They usually become less painful as a woman ages.