Unfortunately, the exact cause of fibromyalgia is not known, but there are some ideas as to what may cause it. Some doctors and researchers think there may be a genetic cause to fibromyalgia. There is a possibility that serotonin receptors and transporter genes may be the cause. Serotonin is a chemical that sends signals through our nervous system. It is thought to be active in constricting smooth muscles, and it contributes to wellbeing and happiness, among other things.