Weight gain is a common side-effect of SSRIs in general - paxil in particular. I have friends who lost weight on paxil with a strict diet and exercise program. But I also have friends on paxil who could not lose the weight no matter what they did. The weight will come off when you discontinue the medication - or switch to a medication that has less of a chance of causing weight gain (ie. Prozac). Serotonin is a neurotransmitter involved in regulating mood. It tends to have a sedating effect that can interfere with your metabolism and make you feel more hungry (especially for sugar and carbs). Paxil is supposed to be the best medication for anxiety and panic, but there are other medications that may work equally as well for you without the dreaded weight gain side-effect. You should speak to your doctor about this. But whatever you do, do not just stop taking the Paxil. All SSRIs can cause withdrawal symptoms, but Paxil's are notoriously more intense because of it's short half-life.
For some reason Paxil seems to be the one SSRI that causes this massive weight gain.
If you don't like the weight gain, then don't EVER let a doctor talk you into taking Remeron. It is the king daddy of weight gain antidepressants. It makes Paxil look like a diet pill.
What about maybe switching to Celexa? I know many people that find it just as effective with no weight gain issues.
I was on Celexa and gained 40lbs in 6 months.. I'm sure we are all different, but Celexa made me gain weight too. ( and no it wasn't because I sat around and ate all day)