What's the difference between fibromyalgia and an undifferentiated connective tissue disease?

A LOT! Any connective tissue disease (ctd) is essentially autoimmune, in nature, meaning the body is attacking itself. 'undifferentiated' may mean it's not classified or not classifiable, but still bears the hallmarks of inflammation & autoimmunity. No one can really agree what fibromyalgia is or what it's cause(s) are, but it does not appear to be autoimmune. Treatments for the 2 are very different.