You might need a second round of Aclyclovir. Have you seen an internist or primary doctor since getting Shingles? It sounds like it's still active.
What MG drugs are you on? Are you immunosuppressed?
I took a LOT of B12 during Shingles. 8 tablets per day (5000 mcg./total of 40 grams), spread throughout the day. There are many studies about using high dose B12 (not the regular dose) while healing nerves. Japan has done a lot of studies on it.
Don't blame the doctors. If they didn't graduate in the past ten or so years, they might not have done the research to know about this drug. I'll bet the pharmacist knows. Someone needs to decide what is worse or if there is another drug you can use for pain (one that doesn't make MG worse!).
Show your doctors these.
http://www.ncbi.nlm.nih.gov/pubmed/12855353
http://www.ncbi.nlm.nih.gov/pubmed/10918256
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3539726/
There used to be more studies on PubMed, but I swear that the drug companies have forced them down.
Granted, some people can handle the drug. However, if you started having more MG symptoms after you started it, you might be one of the ones that shouldn't push that envelope.
Also, the Shingles might have damaged the nerves in your arm. I suggest talking to Mrs. D (PM her) to see if she has any other suggestions for you.
I really hope you heal up soon! It's an awful thing to go through.
Annie