I gained a lot of weight on Nortriptyline, but slowly, so I really didn't know it was a side effect until I stopped the med and dropped 20 lbs all at once. (Also 10 points on both sides of my blood pressure).

I would definitely see a psychologist first, and let them refer you to a psychiatrist if they think it's appropriate. Psychiatrists don't do therapy and most of them don't do in-depth diagnostic testing - basically they're just drug pushers. Psychiatrists have their place, don't get me wrong, but IMVHO they should not be the first mental health professional anyone visits. LiSWs and the like can be very helpful for counseling also, but the trouble is their qualifications vary hugely, and most of them aren't qualified to do testing, either.

With a psychologist you know what you're getting - someone who's spent a great deal of time studying exclusively mental health and/or development, who can and probably will have you do several hours of testing before your first session so that they can cut through to issues you may not be consciously aware of, or that you may be reluctant to raise in early sessions. If they wind up referring you to a psychiatrist, then you know you're seeing someone that at least one colleague has a high opinion of.