The spinal cord carries nerves that extend from the neck down the spine into various parts of the body.  The nerves can be affected when a disc (the cushioning material between the vertebrae) bulges out and presses against a nerve.  This pressure can cause a tingling and or pain in the back of your buttocks down the back of your legs and even into your feet.  This pain is usually associated with the sciatica nerve (also called the sciatic nerve).  I suffered with this pain for many years.  The best way I can describe it is the sensation of someone holding a red hot iron against the back of my leg for 24 hours per day.  It is the most excruciating pain I have ever felt in my entire life.