What Causes Weight Gain After Taking Remeron?

I lost 70 lbs., while on Remeron. But then plateaued. I needed to lose another 20 pounds to be at a healthy weight and decided to go off Remeron. I later experienced a severe panic attack and my doctor suggested i go back on Remeron which i did. I ve now gained back another twenty pounds and am worried it will continue. Is there another medication that is similar but does not have the side effect of weight gain?