Abby, something just popped into my head when I was reading one of your posts. Forgive me if we have already discussed this (me, brain fog).

Polymyositis seems to fit what you described. Here is a quote from the Mayo clinic website:
"Progressive muscle weakness is the most common polymyositis symptom. It typically affects the muscles closest to the trunk, such as those in your hips, thighs, shoulders, upper arms and neck. The weakness is symmetrical, affecting both the left and right sides of your body, and tends to gradually worsen. "
http://www.mayoclinic.com/health/polymyositis/DS00334


I remember a blog I found years ago -- when I was doing MG "research". The guy talked about searching forever for a doctor AND a diagnosis. Apparently, bloodtests do NOT always show antibodies with this disorder -- and it has a LOT of the same symptoms as MG.