Welcome to NeuroTalk:

I think your problem is complex.

Heart rate will go up if blood pressure is low. This happens with some blood pressure meds. But the beta blocker may make this less obvious.

Beta blockers also cause broncho-constriction, as a side effect, and for that reason, people are often taken off of them if they get short of breath.

The TCA antidepressants, also have anticholinergic actions in the body and affect the autonomic nervous system. They also affect the heart conductivity in some people. This may be additive with some other drugs which do the same things.

This is a website that explains it: There are lists of drugs there that interact with each other, like nortriptyline (and also amitriptyline) See if you are taking anything else:

http://www.qtdrugs.org/

This drug checker is good too:
http://www.drugs.com/drug_interactions.html

Also, being low in magnesium and/or potassium will also affect the heart.

And then there is the neuropathic autonomic possibility.
But first go thru the above things I listed, and see if anything changes.