When I first started having MG symptoms, I also had terrible abdominal pain. I was checked for gall bladder problems, appendicitis, colon issues, and everything else that they could think of. I never really got a diagnosis. The majority of the problem spontaneously went away. I don't know if it was MG related or if I had some sort of undiagnosed disease.
I have some weird muscle discomfort in my upper abdomen that I guess I am just going to have to live with.