Dear friend, welcome to HCM.
you could have guessed it right. it could be MG (myasthenia).
is the weakness increased on activity and better with rest?
is there worsening on raising both arms together above shoulder? does this cause breathlessness too?
is there double vision
are lids drooping?
is swallowing difficult?
myasthenia many times is associated with thymoma
(a tumour in the upper chest) which also can produce symptoms.
pl. see your neurologist/ physician soon and get evaluated to confirm or rule out myasthenia, as you had it long back.
you would be evaluated with antibody tests, NCV/ EMG studies, CT scan
of course there may be differential diagnoses.
I hope to have guided you in right direction.
take care and keep me posted on your progress.
good luck !