I don't really have any advice about the weight gain, but I can tell you that it happened to me too.
I've always been faily slim and luckily have never had to really worry about dieting and such.
Until Celexa. I just "weaned" off of it, but I gained a whooping 25 lbs on it in 6 months! I am beginning to lose some of it, but I am currently at my pregnancy weight. Talk about humiliating!
Right now I am on Prozac & Wellbutrin, neither of which are noted for weight gain, and I'm just praying that I will get back to my normal size.