Hi Anna,
Welcome to our little corner of the Internet. There are lots of MGers who are diagnosed based on symptoms alone. Researchers are still finding new anti-bodies that cause the condition and there are no new tests yet. For such a little thing, the neuro-muscular juncture is pretty darn complicated
What doseage did your neuro set your mestinon at? I'd take the first dose as soon as you get the pills from the pharmacy if I were you. If it is MG you'll be quite amazed in about 20-30 minutes from the first pill. Mestinon doesn't build up to its effectiveness. The three times per day thing is because it wears off in your body so quickly.
I hope you find your answers and a treatment for what you're going through.
Best,
Brian.