I am only in the process on getting diagnosed with MG but I was diagnosed with fibromyalgia many years ago.
Studys say that autoimmune disease comes in packs and they both are considered to be one. If my MG comes as a positive - which my neuro says yes but I want another opinion - it would be my fifth autoimmune disease.
But from my research, many docs do not consider that MG causes pain... while it seems that many with MG do seem to suffer with it. I have no idea if it is a separate issue - i.e. fibromyalgia - or is the aches associated with MG.
There are supposed to be specific pain points associated with fibromyalgia. So for a formal diagnosis, my rheumatologist documented the pain with those points and did blood tests. Hope this helps.