What could be the cause of pelvic pain and cramps which come and go?

Normal hormonal changes in the menstrual cycle are a likely explanation. It would be useful to consult a gynecologist. Depending on the severity of symptoms, your doctor could prescribed medication to control symptoms or at the other end, medication can limit your periods to once a year. Get HPV vaccine. Wish you good health!

Any organ in the pelvis- uterus, ovary, bladder, colon can do that. If it persists you will have to be checked by your doc for without that a diagnosis is not possible.