Hi and welcome to HCM.
It sounds like ovarian cyst and it can be small which then ruptures ant it doesnt need to be seenon ultarsounds. Also such periodic pain may be caused by endometriosis or hormonal imbalance. If nothnig cant be found and pain persists laparoscopy may be considered. But,also this pain doesnt need to be related to your pelvic organs. It can be caused by ureteral stones ,uroinfection or diverticulosis. So,go step by step and rule out gyn disorders first. WIsh you good health.

Hi! As you have had severe bout of pain, and the pain is recurrent, it can not be overlooked as normal. Many times, ultrasound or x-ray may not be able to detect few of the pathology which have not attained detectable size by ultrasound.
I would suggest either to go for a repeat CECT scan of your abdomen which is non invasive. If this is also inconclusive, then you have to consult a laparoscopic surgeon in your area for a diagnostic laparoscopy, which can conclusively detect the problem and possibly it could be therapeutic also depending on the pathology detected. Wish you an early recovery.