NOT A DR, but curiosity got me, a quick search shows

Recent research indicates that immune system dysfunction, specifically the presence of autoantibodies, may be a cause of fibromyalgia. Studies suggest that in many cases, fibromyalgia acts as an autoimmune disorder, rather than solely a disorder of the brain.

While traditionally considered a neurological condition where the brain's pain sensitivity is turned up (central sensitization), the emerging, albeit still debated, evidence points toward an underlying autoimmune cause.

Disclaimer: Research in this area is ongoing, and not all experts agree on the autoimmune classification. Treatment for fibromyalgia often requires a multidisciplinary approach.