Although Annie answered your question, I wanted to say that I experience the same thing. Generally I don't experience pain but my muscles do easily get cramps from overwork (or normal use if I don't rest often) which rarely happened to me before I got MG. It can take up to a week of rest for my sore muscles to stop hurting, but once I am rested the pain goes away.

Pain from using weak muscles is absolutely a SECONDARY symptom of MG.