Yeah, muscle pain is not an "official symptom" I suspect this is because you cannot feel the actual attack on the neuro-muscular junction as there are no pain receptors there but it certainly seems to be a physiological symptom to me.

There are about 630 muscles in your body. There are 13 muscles in a leg. If, like me, your smaller muscles are affected first then the others have to compensate and so they get worked harder and a hard worked muscle gets sore.

I know my left side is effected worse by the MG and my left side shoulder, back & neck certainly feel more sore more frequently than my right side.