I lost 7 lbs initially. I talked to my psychiatrist about it and he said that it was only due to reduced appetite which is normally temporary. He said it shouldn't have any effect on my actual metabolism or anything like that.
Have you noticed an increase in appetite or changes in your eating habits? Increased appetite can also be a sign of depression. I'd definitely bring it up at your next appointment to see if they feel your zoloft needs to be adjusted.
I've gained 20lbs since starting in Feb. Side effects vary person to person, though.