What i have told is primary fibro is when there is no known cause and secondary fibro is fibro after trauma or something like a autoimmune disease.

although this can be misleading in that fibro has so many symptoms that many autoimmune disorders can be missed or not found till years latter.

I my case the autoimmune disorder was not found till 14 months after the fibro started and the very very rare autoimmune disorder i had was a asymptomatic type (castleman's disease) and was found by accident during a CT scan.

One small study of people with primary fibro for over 15 years found that 95% had been DXed with a autoimmune disorder years after there fibro started.
The doctors that did the study believe that these people had the autoimmune disorder all a long but were asymptomatic or the symptoms masked by the fibro and the autoimmune disorder had been missed.

If this is the case then fibro would be a Secondary Autoimmune Disorder.