Different meds affect different people differently. What is weight gaining for some isn't for others. You also don't state if you stopped Paxil abruptly and switched to Zoloft, or tapered slowly off Paxil first before switching. Because you are describing Paxil withdrawal -- it's one of the most difficult meds to stop taking and must be done carefully. Just switching to another ssri won't necessarily do anything about withdrawal from a different ssri. If it is withdrawal, of course going back on Paxil is the remedy, but that doesn't fix the weight gain problem, which is itself a significant health hazard. Hard to imagine any psychiatrist or doctor allowing you to gain 100 pounds without having said something about Paxil being notorious for weight gain. If you choose to go back on it, you might try consulting a book called the Anti-Depressant Survival Guide by Hedaya -- it suggests dietary changes to help with this kind of side effect. It's not a great diet, but it should give you some ideas. Good luck.
Yes, it's safe to go back to Paxil. Both of them have side effects of weight gain, but go with what makes you feel best. Some people I know follow the glycemic index for eating - - it does work! Best wishes!!
Zoloft causes weight gain just as much as Paxil. Go back on the med that works for you! Change doctors if they give you fuss.