Multifactorial. FM "umbrella label" for several disorders, each defined by unique & distinct set of identifiable causal factors. Disorders with this label share similar symptom constellation. Unifying symptoms are chronic widespread pain, non-restorative sleep, depression & cognitive disarray. Causal factors include femaleness, childbirth, joint hypermobility, obesity, biomechanical injury & prolonged inactivity.
If fibromyalgia? Fibromyalgia is characterized by a constant dull ache, typically arising from muscles, occuring on both sides of the body, both above and below the waist. Pressure on tender points (see diagram) also characterize the condition. Sleep is frequently disrupted by pain, and fatigue, anxiety, depression headaches abd irritable bowel syndrome may co-occur.
Don't know. That abbreviation is neither specific nor widely used. It can refer to illnesses ranging from fulminant Meningococcal septicemia to fibromyalgia syndrome. I've even seen it used for false memory syndrome.