Pain on left side below rib, toward back. had mri, blood work & urine tests, all was ok. 3 weeks later, still pain, doesn't feel like muscle strain.

Anatomy is important: You did not say what MRI your physician arranged for you, abdomen? Spine? Or how the pain varies with your activities. Please see a physician who can perform a detailed structural examination to guide your posture, work tasks and exercise habits. A common source of pain in this area is scoliosis, which is seen best by x-ray.

Quadratus lumborum?: Assuming that your MRI was a normal lumbar mri, that you do not have a rash over the area (shingles), and that workup for a kidney stone was negative, then i would consider the possibility of quadratus lumborum pain. It starts deep in the flank between the hip and ribs, and can radiate to the back or groin. Consider seeing a physiatrist or pain physician to verify.