Taking 1 (60mg?) tablet of Mestinon will have an effect within 30 minutes, and it will be out of your body within 6 hours.

If one tablet is making you feel worse.... go back to your Doctor. 1 tablet should not be enough to be an overdose, but if it is making you worse then it is clearly not working for you.

That may mean that you don't have MG. Or it might mean that you need a different medication. See you doctor.