The NIAMS says fibromyalgia is a syndrome versus a disease. A syndrome consists of many medical issues and symptoms simultaneously.
NIAMS states, "A doctor familiar with fibromyalgia, however, can make a diagnosis based on criteria established by the American College of Rheumatology (ACR): a history of widespread pain lasting more than 3 months, and the presence of diffuse tenderness." In order for a diagnosis to be established, you must a have pain in 11 areas of the 18 ACR assigned areas.
It is unknown what causes fibromyalgia. For some fibromyalgia symptoms may occur overnight and for others the symptoms occur over a period of time. Some have said their symptoms occurred after a traumatic incident like a death or accident.
People who have family members with fibromyalgia are also likely to develop the disease. Individuals with spinal arthritis and lupus (SLE) may develop fibromyalgia.
https://www.empowher.com/fibromyalgia/content/do-i-have-fibromyalgia