What is the cause of a severe lower abdominal pain with an expansion of the scrotal sac?

Hernia/torsion: Please go to er because your strangulating inguinal hernia can be the cause. You need ct of abdomen & pelvis or at least a sonogram depending on location and availability. Good luck.

Possible hernia: The symptoms you describe could be a hernia. Nonetheless, that is a guess on the computer. You need to get an eval as soon as possible for a possible hernia, or other causes, ... As if it is a hernia and your scrotum is getting larger, you want to assure it does not get "strangulated." good luck.