I had almost the exact same experience only with my left leg instead of shoulder/arm. I was on crutches with an immobilizer on for a few weeks after tearing a bunch of stuff in my knee. I remember being completely amazed at how fast the muscles, specifically my quad, just disappeared. My leg turned into jelly. The best part about losing muscle in one leg was occassionally I would just fall flat on my face
. After the immobilizer came off but before the muscle built back up I would be walking and my knee would involuntarily bend when I put weight on it, I found the situation quite funny, especially when it was in front of others. Also, after years of favoring my right leg for all activities it appears I may have some problems with that one now. My good leg has become my bad leg and vice versa, so watch out for that.
Anyway, just wanted to further confirm Meg's experiment, not that it needed any confirmation. And BTW, good luck with the rebuilding and try not to push yourself *too* hard.