Hi and welcome to HCM. Thank you.
First you should find the exact cause of large extrarenal pelvis. It can be found in healthy people but also some ureteric or bladder obstruction may be cause. This should be investigated by intravenous urography and then treated. Possible causes are stones, tumours, abscesses and extrarenal masses. If you dont have bad renal function findings urea and creatinine
then you shouldnt be worried. But this pain can come from kidney
disorder. Wish you good health.