I think you need to carefully identify the pain as joint or muscle. You can have pretty severe muscle pain with MG particularly when the muscles are extremely fatigued and will not work. MG does nor cause joint pain and swelling like RA. And is the inability to move due to the pain or perhaps you cannot move the muscle because it is so fatigued and when you try it is very painfully ( which is more like MG).

Before I started mestinon and prednisone and I had severe episodes of MG, I could not move my leg muscles. Of course I would keep trying and they would barely move and not really in the way I wanted them to and this would be extremely painfully. And I would often fall. That is what MG is like. Also Ice helps relieve the muscle ache and fatigue of the muscles with MG. Heat makes them worse.

I hope this helps
kathie