(specifically chapter 52 pages 461-462)
This is a text book that I used while in college and have referenced many times since. I will use the text to offer some further understanding to pain. The following has been re-typed exactly as it is in the text book.
There are several causes of pain. Perhaps the most common is muscle tightness. You can test this theory by making a tight fist and holding it; after a few seconds, you will begin to feel discomfort. This is caused by muscle spasms and by the resulting decreased blood supply to that muscle. As the oxygen supply to the muscle decreases, the discomfort or pain increases. This same sort of muscle tightness often occurs in pain.
Pain may also be of psychogenic origin; that is, it originates in the mind and has no identifiable physical cause. Psychogenic pain can be as severe as pain from a physical cause. Usually, pain has both physical and psychogenic causes.