Fibro is considered a chronic syndrome, not a disease as such. It’s a syndrome because it’s a collection of symptoms.

From what I understand it is not autoimmune. It is neurological and muscular. But, I am no expert.

Yes, I’ve had costochondritis that lasted for years. Honestly diet and exercise help keep it at bay. I walk regularly and do stretches too.

My understanding is that fibromialgia is not considered as an autoimmune disease based on current research studies.
"An autoimmune disease is a condition in which your immune system mistakenly attacks your body. "
"In laboratory tests, people with fibromyalgia typically have low inflammatory markers, while autoimmune diseases are associated with high inflammatory markers."
https://www.verywellhealth.com/is-fibromyalgia-...

However it could be confusing that fibromialgia patients often have comorbid conditions which are often autoimmune such as rheumatoid arthritis or lupus, IBD or MS and their symptoms are overlapping with fibromialgia symptoms. So you can have Fibro and other diseases unfortunately but fibro on its own would not elevate your inflammatory markers.

From my research, in general, it is not accepted as autoimmune just yet. But a University in the UK and one in Sweden both got similar results, when they tested on mice, that fibromyalgia may be related to the body’s antibodies. Don’t know if that helps. I am new to this so I don’t really know how it works. I think my own doctor said it was not an autoimmune disease because I might have asked that question directly.