Hi Eva,
Nortriptyline is of the older
tricyclic class of antidepressants. They are often prescribed in light doses (like yours) for chronic & neuropathic pain, particularly at bedtime to help patients sleep the night through.
I'm taking amitriptyline 10 mg. myself for just that purpose, and it works quite well as long as I go to bed within a few minutes after taking it. If I wait more than about an hour, it sometimes doesn't/will not work for me. It's worked for me from day 1, so if it doesn't help you any within the next week or so, I'd call your doctor & let him/her know. S/he may try switching you to another or upping the dose.
Hoping it helps you,
Doc