The cause of back pain that is severe whether on the right or left side can most likely be traced to a problem, which is affecting the sciatic nerve. The sciatic nerve is the longest nerve in the body, it begins at the lower part of the spine and radiates downward through the buttocks and runs down the legs all the way to the toes.
When the sciatic nerve has pressure placed on it, it can cause the back pain, (commonly referred to as sciatica) which at times can be severe and debilitating. It can be on the right or left side of the back and sometimes both sides. It can be localized in the lower back however, the majority of the time it radiates downward through the buttocks and down the back of the legs.
Most problems with the sciatic nerve that cause the pain known as sciatica is related to damage to discs in the spinal column or muscle damage to the Piriformis muscle. The Piriformis muscle is the muscle that attaches the lower back to the thigh bone. When the muscle becomes damaged and swells it places pressure on the sciatic nerve because the sciatic nerve runs below this muscle, thus causing back pain.
Most people, when the back pain is severe and debilitating, will tend to try many different positions to sit or lay in to try and ease the pain, or (as I did for many years) lay on a heating pad to ease the pain. When the pain first flares up, it is because of inflammation, heat only causes more inflammation, it feels good while applied because the heat numbs the pain receptors which send signals to the brain. Once the heat is removed, the pain is worse because the heat worsened the inflammation.
When the back pain is severe and is being caused by a problem with a disc or discs in the lower back sitting is the worst thing you can do. Sitting, compresses the spine, and causes more pressure on the sciatic nerve, short walks will help more than sitting.
Whether your back pain is being caused by a damaged Piriformis muscle or damaged disc, the pressure that either condition will place on the sciatic nerve causes the nerve to become inflamed thus causing your back pain, severe or debilitating.