A small stone is possible, however with a negative ultrasound and CT scan, that would be less likely. The CT scan is a pretty sensitive test for kidney stones.
If the pain continues, a contrast CT can be considered to look for evidence of a hernia. If you are male, sometimes prostate or scrotal problems can radiate to the groin area.
If the tests continue to be negative, you can consider musculoskeletal causes that can lead to the back pain. A back MRI can be of use if this is suspected.
These options can be discussed with your personal physician.
Followup with your personal physician is essential.
This answer is not intended as and does not substitute for medical advice - the information presented is for patient education only. Please see your personal physician for further evaluation of your individual case.
Kevin, M.D.
kevinmd_