In a particular position there is severe pain in the back. above left hip area. how should it be treated?

Diagnosis first: Posterior (back) hip pain is rarely from the hip. The area of pain you're describing could be radicular or sciatica (e.g. Pinched nerve in the back), referred pain from the back (e.g. Facetogenic pain), priformis syndrome, si joint dysfunction, cluneal nerve entrapment, iliolumbar ligament enthesopathy, etc. You really should see a doctor for an accurate diagnosis & proper diagnosis.