*stands on soapbox*
I've said it before and I'll say it again. I think IBD patients only lose weight when their situation is bad enough that they don't want to eat.
If you are getting treatment and you feel like your symptoms are reducing, you might start eating more than you did before. I think it's a natural response to "stock pile" or feel like you deserve to eat while you feel good enough to do it. Not that that's what is happening with you.
I think it's an extremely common misconception that Crohn's makes you lose weight or that you shouldn't be able to gain weight. There are so many reasons that Crohn's makes you gain weight. We modify our diets to be things that are "easy" to digest, but they are also high in calories. We are often exhausted and find it difficult to exercise. We are frequently put on drugs that cause weight gain (PREDNISONE).
In short, no, you are not alone in gaining weight. In fact it seems to happen to a lot of us.
*steps down from soapbox*