I've had total knee replacement surgery on both knees and now am having severe pain in left buttox and lower back, what can I do? Pt hasn't helped

Spine evaluation. Spine pain radiating to the buttock region is a sign of nerve compression in the spine coming from either a herniated disc or arthritis in the spine. A spine evaluation is needed.
See your surgeon. It is quite likely that the pain you are experiencing in the buttock and lower back are completely unrelated to your knee replacement surgeries. If your pain has not responded to physical therapy, a thorough evaluation of your lower back (lumbar spine) is indicated.