MG is mainly about muscle weakness that is variable. If it would help to have the EMG redone to reassure you, then maybe you should do it (by an expert). Or you could see a neuro-ophthalmologist to have them objectively show you if you have double vision and why. It's easy to freak yourself out when you feel so bad and start thinking because you have ALS. An MG expert would more than likely know the difference between the two.
Maybe if you sought out the opinion of an MG expert, you would not only feel better but have more information you may need about the course of the disease, treatments, etc.
MG is an autoimmune disease. Mestinon only helps with symptoms; it does not suppress the immune system that is reducing your acetylcholine and making your muscles weak. Some studies do suggest it does help with the immune system but not enough have been done. The increase in symptoms may only mean that you need another treatment, as Deb said.
Please call your neurologist's office and make an appt. to discuss all this. It's hard enough dealing with a new diagnosis let alone worrying about it being ALS instead or progressing.
Annie
Here are two websites so that you can read up on MG:
www.myasthenia.org
www.mdausa.org