I have a left back pain and also left leg and buttock, it burns and electric shock.

Pinched nerve: Whether from back ( from pressure on nerve from a herniated disc for example) or from spastic muscle in buttock putting pressure on the nerve, should help to take an anti-inflammatory medicine like Ibuprofen if you don't have kidney problems or allergic. Look uppiriformis muscle stretches and strengthen abdomen and back muscles as part of a back program with physical therapy a good place to start.