Felling dull pain in right hand testicle/lower back/leg. Had all tests- testy, kidney ultrasounds/mri of pelvis/urine & blood clear. What could it be?

Urine culture? You did not say if you had a urine culture done. Sometimes this kind of pain can be due to an infection that may not show up with routine urine cultures. An example is nongonoccocal urethritis or prostatiis. Some docs might try a course of antibiotics and see if that helps the symptoms, since cultures can be expensive. Discuss this with your doc or see a urologist.