Pain in back, possibly along sciatic nerve, when on period. most definitely not cramps. occurring for 1+ years. only lasts for duration of period.

See your doctor: Given the duration of symptoms, you need to be seen by your doctor who can examine you and possibly recommend further testing in the form of imaging.

Check it out : If no cysts or internal issues, may be helped by chiropractor .

Likely related: Your comment of only having the symptom during a period tells a lot. Hormones and fluid changes account for symptoms sometimes nsaids can help and are recommended and if tender at your psis (posterior superior iliac spine) your si joint could be the culprit

Endometriosis: Endometriosis results when the lining of the uterus escapes through the fallopian tubes and attaches to intraabdominal structures. Some women with endometriosis can develop sciatic symptoms if the tissue attaches to the posterior wall of the abdomen where the nerve plexus that forms the sciatic nerve resides. Supression of menses with birth control pills may be of benefit or laparoscopic surgery.

Referred pain: Likely that this is whats called referred pain secondary to your cramps and predictably happens with your cramps only. Nothing to be concerned about unless it changes or lasts longer to your cramps or is not associated with menstruation.