Most definitely talk to your doctor about this. It's listed as a "severe" side effect for this drug. You want to make sure this isn't the start of hives. that can get in your throat, etc. so you do have to be careful. If your doctor thinks this is just very mild, you can probably use a cortisone cream that is sold over the counter that will help with the itching. Just be careful. I sure hope this all works well for you since you are having such great success with this. Below is the link about nortriptyline that I got my information from.
http://www.drugs.com/sfx/nortriptyline-side-effects.html
Sherrine