Fat and Crohns

Once you have lost some of your fat reserves, it will take a long time to fill in. I read that your body first uses the fat and puts it in much needed places, apparently the face gets it last. It may take you several months to fill in, depending on your natural ability to put on weight. If you've always been thin, then your metabolism may be higher and burn off calories quicker.
When I use to get my flares I would lose it and then regain it when I felt better, but with the last drug they've tried me on, I lost some permanent weight, to the point that I was getting muscle wasting. It took me a couple of years to regain some of it. I'm still trying. I think your weight will increase eventually and you will fill in, just keep on trying by eating a healthy diet. Best of luck.