Tracy, what you say is very interesting. Because sometime ago, I discussed the fact that MG patients do not only have motor problems, but also sensory problems. We lose our ability to automatically know what is heavy or light or how strong our muscles are and we have to find ways to compensate for that (just like a person with sensory deficits).

There is a possible complicated physiological explanation for why that happens.
A rather simplistic explanation is that our brain gets confusing information from our muscles because on the one hand it has to fire more to get it moving (which makes us think that something is heavy) and on the other hand the tone of the muscle spindle is decreased (which makes us think that something is light).

I know that in the early days of my illness I would have no idea if the cup I tried to pick up was heavy or light. It was a true "alice in wonderland" experience. I felt as if I lost the ability to navigate. I had to teach myself how to know. I eventually learned to do it so well that I could tell what my peak flow is going to be quite accurately before measuring it. (even I was so surprised by this ability).

But, if I don't concentrate on what I am doing or am distracted by something, I totally lose my ability to know how strong I am and exactly like you describe I can be surprised at how strong or weak I am. I can choke on my food if I forget to concentrate on my eating. Probably just like a blind man would if he forgot to use his cane or other senses that compensate for his disability.