It probebly is a mixture of both. I happened to gain some weight on Lexapro. And now I am on Celexa-I gained weight with this one too. The main thing is that you are feeling better - the weight issue will taper out.
Keep an eye on your thyroid. It is very common to become hypothyroid when you quit smoking. If you get tested get the full test not just the TSH. Get a copy of your results & post them on the Thyroid forum. Most Drs. don't know squat about thyroid. They will likely tell you that you are within normal range. That does not mean that you cannot be hypo. My daughter & I have been living this nightmare since we stopped smoking 6 months ago. We both became hyop within a month. We have finally found a good Dr to treat it.
BTW, I am on this forum because of anxiety that has developed as a symptom of hypothyroidism. I have never suffered with anxiety in my 52 yrs. until I became hypo. I know that when I get my thyroid meds optimalized that the anxiety will be a thing of the past.
Congratulations on not smoking. That is a hugh accomplishment.