Troy,

I found this: “Myasthenia gravis itself does not cause pain, but the weakness may lead to non-specific aches and pains. For instance, neck pain may occur because of weakness in the neck muscles.”

which is part of this MG article

I’m having sharp pains in my left should when my arm is in certain positions. My guess as to its cause is that my muscles are weakened by MG and are no longer holding my bones, muscles, etc. in their proper positions away from the nerve.