What Causes Muscle Twitching In Legs While Having Myasthenia Gravis?

I m noticing my lower left leg is becoming smaller than my right. It looks thinner anyway, and measures 3/4 smaller than the left leg. This is in the area from the lower calf to the ankle. I have pain in the leg and foot after being on me feet even for a little while. For the past year I ve felt that leg was weak, or clumsy, and I have pain - or irritation - in one spot at the base of my calf, toward the front. I ve also had severe muscle cramps. A couple months ago I began to have muscle twitches all along that leg, especially in that area of my lower leg, but in other places also. The twitching happens even when I m not moving. I have Myasthenia Gravis but this type of pain and other symptoms are different than I have experienced. Is the 3/4 difference what is meant by atrophy ? And what might he causing this?