I have MG for 3 years now.
First year I got the flu shot, but was not on immune suppressants. I got a real flare up of the disease. But because it was my first year of MG, it was possible it was just a coincidence.
Second year, I was on prednisone, methotrexate, etc, I got another flu shot.
Well, there was another good flare up: I couldn't say one word in 3 weeks, and was on liquid food.

I am sure it was because of the flu shot. My docs told me this could happen, because there is something in there that activates your immune system.
This is my first year with mg, and withouth flu shot. Just see what happens I'm still on immune suppresants, and also have other diseases that have recommondations to get the flu shot. And I still have breathing problems etc due to MG, so if I do get the flu....well, just wait and see I guess.

By the way, if you are seronegative, then it may be a different story. But for me it was: flu shot -> activation of immune system -> more anti AchR -> more weakness for 2 - 3 weeks.