I'm reading Healing Back Pain: The Mind-Body Connection by John Sarno. It is fascinating.
Dr. Sarno describes a concept he calls paranightmare: when you are about to have a strong negative emotion, sometimes the mind changes it to a physical pain, so as to avoid the emotional pain. Dr. Sarno says that chronic pain is caused by repressed emotions in the unconscious mind.
(I created this page since a Google search for paranightmare didn't show any useful results.)
More info about back pain here.