What Causes Pain In Hips And Buttocks After A Back Muscle Pull?

Pulled a muscle on the left side of my back while deadlifting 300lbs. When it happen I felt a small pop. I went to the doctor and I did a. MRI. The results was normal..tissue, nerve, muscle and spine. However I still feel some pain radiating in my hips and buttocks at times. The pain comes and goes. My back muscle whicH was strained is the lower back. Next to where the kidney located. The doctor told me that it s a muscle problem..no need to be worried. Can u enlightened me as means of second opinion