Wow--the same thing is happening to me, and nobody can explain it. I was diagnosed with CD in 1999. In 2001, I suddenly lost 10 pounds--and then, after my doctor put me on a low fiber/low residue diet, I dropped another 10. I was probably too thin, but I felt really good. Over the past year, without changing my eating habits at all, I've been putting on weight steadily but gradually. My gain now totals about 18 pounds--and I'm miserable. Nobody can explain to me why this is happening, and I agree, it's frustrating.
The only interesting idea came from a personal trainer (not mine--I can't afford one!) who said that my body had been starving itself for so long that my metabolism is now slowed way down, meaning that I'll gain weight if I eat anything like a normal diet. If that's true, what the heck can I do about it? I was always told that the only good thing about having Crohn's was that I'd never again have to worry about my weight. Clearly, that's a myth.