I really wish I had the same issues. I am struggling to take weight off. I am 5'5" and male and should be 170 pounds accounting for my body type but instead I am over 200. It's a real struggle. My blood pressure has crept up, knee issues have developed (osteoarthritis not helped by the extra weight), sleep apnea and other issues have developed, and some ED issues have arisen I didn't previously have when my weight was lower. This is all not good. It's metabolism and not the J pouch, although how your body's metabolism adapts to the J Pouch may be another issue.
If I eat badly I immediately gain weight. I am talking carbs and sugars. And I also struggle with my portion control. Part of that problem is that I am a foodie by nature and have great difficulty controlling that tendency to curb my curiousity on trying new and different foods.
Part of my problem is a lack of activity and I suspect if some of you dial back your activity and become couch potatoes for a few weeks you will see the weight come back. I am not advocating coach potatoism long term - just short term to put on some pounds. Grab a bad of potato chips and some beer and watch the basketball games the next two nights. Throw in a little Netflix. No activity other than sitting on the couch!!!! At halftime of the basketball games, run out to the local convenience store and pick up a pint of Ben and Jerry's ice cream and eat it in one sitting. Do these things and if yo don't gain weight, then there is a serious issue and you need to see doctor and nutritionist.