The benadryl might, as it can, be causing you worse tachycardia?
Diagnosed by tilt table with P.O.T.S. in 2004, I suspect I have that form of P.O.T.S. (hyperadrenergic) due to various abnormal catecholamine type numbers in a number of lab tests. Medicines that were tried and discontinued for either side effects or not helping- lopressor, florinef, midodrine, and verapamil. Currently, I am not being treated by any physician for my P.O.T.S..