I lost a lot of weight with UC and gaining back after the surgeries was a bit of a struggle. I had and obstruction and ileus post opt and basically couldn't eat solid food for 3 weeks. I would say losing at least some weight seems to be the norm with this surgery even without complications, but regardless, your focus now should be on trying to put weight back on. At my documented lowest I was 86 pounds (I'm 5'3). Prior to getting ill, I weighed around 120. My GI wanted me to drink 6 Ensure a day (I was lucky if I could get 2 down).
What helped me most was seeing a dietician; I was lucky enough to have access to one through the hospital's after care program (called CCAC in Ontario). Anyway, the suggestion by the dietician was to eat 6 smaller meals a day rather than 3 large ones. It amounted to basically eating something about every 2 hours. Foods high in protein were recommended. On this regime, I gradually gained back the weight.
At the time of my surgery, Greek yogurt wasn't very popular, but it's everywhere nowadays, and as far as j-pouches go, I think it's the perfect food. It's high in protein for one thing, many varieties also have high levels of probiotics, also great. If lactose is a problem for you, it's lower in lactose than other types of yogurt, and more and more often I am seeing lactose free varieties at the store.
Other things you could try that I was able to tolerate post surgery: smooth peanut butter, avocados, scrambled eggs, cottage cheese, oatmeal, canned tuna, Kraft Dinner, rice, grilled cheese sandwiches, and potatoes.