AnnieB3 has had some of the best descriptions/answers about this.

In a nutshell - it seems everything and anything can effect the way acetylchline and acetylcholinesterase behaves in our bodies (within the day or over days - even with meds). There is no predicting the cause/effect with repeatable reliability.

That's why this MG is such a puzzle. Can't count on what the day will bring (even with meds).