Fibromyalgia and arthritis both cause dull pains, fatigue, and sleep disturbances. However, arthritis is an autoimmune disorder and, while the cause of fibromyalgia is unknown, it’s likely a neurological condition. The disorders also progress differently. Fibromyalgia is characterized by consistent pain, though it may worsen with stress or poor sleep, while arthritis is characterized by progressively worsening pain flare-ups. Also, joint inflammation causes arthritis pain, while tender muscles and joints cause fibromyalgia pain.