I was on Remeron for bad insomnia myself and yes, I gained weight ! I was eventually taken off of it because i gained 80lbs!!! But, I am on other psych meds too which cause weight gain too. I did have a water retention problem as well and went on??? whatever it was, began with an H, and I did loose weight. So, you may be getting some retention even though your drinking alot of water. Also keep in mind that if you are having constipation problems, feces are not light and while stored in the body do have weight to it.