It's possible that the diagnosis was wrong, though it's also possible that you have another disease, side-effects of medication, or worsening of your MG

Do you have positive antibodies? And does mestinon work for you? On what ground did they diagnose MG?
That way you can kind of see if this is a correct diagnosis, especially if you have antibodies.

Are you only taking Mestinon? If so, maybe you need more medicine and it's your MG worsening. Have you been to your neurologist? If not, probably best contact him/her.
Are you taking other medication for your MG besides mestinon? If so, contact your neurologist and let him/her check side-effects related problems specifically.

And another option is indeed that some other problem is causing these symptoms. It's not so easy to find out where to look at, if the blood comes back normal and the symptoms can be caused by anything.
However, some with MG are predisposed to other auto immune diseases like thyroid problems, diabetes, and all kinds of rheumatologist stuff like RA and SLE.
These all tend to give you some of the symptoms like the ones you are experiencing and can be hard to diagnose sometimes.
Maybe see an internist?