I have never taken, and am not finding pyridostymine coming up in a search. Do you mean Pyridostigmine bromide? I see it is used to lessen muscle weakness for people with myasthenia gravis.
Did your doctor explain why they felt the medication was something you should take for your P.O.T.S. (how they thought it would help) or did they say they were giving it to you for something besides your P.O.T.S.?
I've never heard of this drug being given for P.O.T.S. before. I would tell the doctor what it seems to be doing to you with the tiredness- if it is Pyridostigmine bromide, it's a drug that can make you drowsy... perhaps it is not the best medication for you?
Had they tried the usual types of medications for your P.O.T.S. that doctors try, such as midodrine, florinef, and beta blocker medication?