Groin pain is most commonly caused by inguinal hernia
Unless,the hernia is big it is not seen in CT or MRI
So,to confirm it,get Herniography.
Here ,a dye is injected in the peritoneal cavity
in the abdomen and x rays are taken by flouroscopy.The hernial sac is clearly seen.
Another condition you have to exclude is Intermittent testicular torsion
.Here,the testis gets twisted and then gets derotated.This will be confirmed only with Colour doppler at the time of pain.
If you come back with Herniography report,i will clarify further
Wish you good health