Same here, although it is different every month. Sometimes around ovulation I feel almost normal and have to be careful not to overdose on the Mestinon. If I am going to have a bad week it is usually during the week before my period starts.

Something I did notice is that when I was taking Magnesium vitamins regularly, I didn't get the "crash" with my period, at least the weakness wasn't as bad. I am eating more vegetables that are high in magnesium now, and only taking the Magnesium vitamins on occasion, this really seems to help but not prevent this monthly effect. Gives me hope. I wish that my neurologist was trained in the effects of nutrition on MG.