Thanks, Abby!
Joe, if she is doing that poorly, she needs to be in an ER, right away. The inability to breathe well is an emergency. They can do breathing tests, arterial blood gas test, O2, and much more (like giving her oxygen if necessary and writing a Mestinon script). That will tide her over until she can see an MG expert.
After that, contact the neuro that Abby recommended!
With positive antibodies, how can a doctor take away Mestinon?! That's criminal. The doctor could be sued for malpractice (don't tell the doctor or anyone else that, though!). A negative EMG means nothing. A negative doesn't prove or disprove a disease state.
I'm so sorry she isn't doing well. There's no reason for that. Please, have her go to a good hospital close by and get some help. You don't want her to go into an MG crisis! Don't be scared, just take action.

Annie