Yes it is possible , typically people usually lose weight. I have not heard of weight gain but its possible. Just get plenty of exercise and eat clean. Zoloft is generlaly not a drug that will make it hard to lose weight. My guess would be the drug itself has little to do with the weight gain, maybe you are eating more as a result of feeling better. Best thing is to ask your doctor about it. I have been on it before and it actually killed my appetite.
Your husband sounds like a real gem! Giving birth and then suffering PPD wasn't enough for you, you have to put your recovery at risk too? Slap him upside the head for me. :-)
Your health is important and he AND your baby need you healthy. If that means you carry a few extra pounds a bit longer then he'll just have to accept it. He'll be far better off then he'll be if you're too sick to do anything for either of them. Take care of yourself and hug that baby.