45 yo female with sudden onset intense intermittent sharp pain right side of back just under rib cage. what are most likely causes?

Kidney : The kidneys reside on the posterior wall of the abdomen at the level of the lower ribs in a person's back. Intermittent pain in this area is either from the diaphragm, the ribs themselves (as in arthritis) or spasms from a kidney most often caused by kidney stones or infection.

Kidney stone: Although ureteral stone is a possibility, it can be ruled out with kidney stone protocol (no contrast) ct. If negative for stone, consider early shingles neuralgia or simple muscle spasms. Stones in the kidney proper rarely cause severe or sharp pain.