I had a similiar problem - never had to worry about weight gain until my ileostomy. Finally I feel better and I have packed on the pounds. I had no idea how to count calories as it never seemed to matter before. Everything I ate just went right through me.
I recently started weight watchers so that I could learn about nutrition and try to diet. Its worked for me and I have been able to lose 5lbs and counting in the last month. I never thought I would need to diet but it seems to be working.
You may want to talk to your doctor about what they suggest and keep in mind it may take your guts sometime to get adjusted to being disconnected.