I have a lower back pain that is somewhat connected with my right knee... i had an MRI of the back and the knee and x-rays too but nothing is wrong... now i had a kidney ultrasound saying there is a post void residual urine - doctor said its ok.  I really do not have an idea what doctor to see... ive seen ortho, rehab, Gynecologist,
It seems to be simple backache. Cause may be muscle weakness or bone weakness. Got it D3 level may be helpful. If vitamin D is low then take its supplementation, otherwise back exercise will be helpful. You should consult or tho.