Ask your doctor to run a Celiac Disease screen (blood test). The rashes might be a form of Dermatitis Herpetiformis (the "external" manifestation of CD).

It wouldn't necessarily explain the muscle weakness, but a gluten-free diet might ease some of your other symptoms (some people test negative for CD, but are still gluten sensitive).

Many of the MG'ers on here have found a GF diet to be beneficial - it's pretty easy to do, once you know what you're doing.