Fibromyalgia is characterized by a number of symptoms, primarily extensive chronic pain, and is not associated with any specific cause. There are various physical and psychological factors that could be the trigger of this disease.
The cause of fibromyalgia is unknown. What we do know is that it involves an anomaly that results in an increased response to pain.
Contrary to popular opinion, fibromyalgia is unlikely to be a psychosomatic disease. Certain psychological aspects do come into play, however, such as contributing factors or the perpetuation of the condition.
The start of fibromylagia syndrome could be progressive (slow) or sudden. In the case of sudden onset, the disease is related to a triggering event such as stress, a traumatic event, a traffic accident, anesthesia, etc.
For more information you can go to the fibromylagia forum.
Source: UK National Health Service