My brother went away to rehab skinny and he was addicted to opiates as well he came home with some more weight than he got put onto suboxones. I think the weight u are gainin is because you are not on the opiates again, but go to gym or watch you diet and you will be fine, but stay off the drugs !!!
Any opiate can cause digestion and gastro-intestinal issues. Some patients do gain weight while others might have constipation or other symptoms.