I've had the same problem with weight gain issues as yourself. I'm probably medium build naturally, not fat or particularly thin just average really. When I get ill, I lose weight rapidly (and of course not being able to eat anything just adds to that), but when I start getting a little better - as soon as I start eating a little again - my weight increases just as dramatically. I was very poorly before my ileostomy and lost loads of weight, then I had surgery and started eating little bits of food again - my weight ballooned substantially. I'm also immobile because of abscesses, fistualaes and chronic tiredness so it's really difficult to get the weight off again and maintain any kind of normal weight. (I'm 5'7" and about 220lbs ish - my weight's taken in kilograms so I'm doing a rough estimate here!)
My Nutritionalist at the hospital explained the 'feast or famine' nature of my eating over many, many years. I'm literally starving for months and months at a time - then I'm back to eating (all be it restrictedly) for a while. Your body holds onto any calories it gets and lays it down as fat - so that you survive the next famine! This is particularly true if you've been overweight before the Crohn's (because the fat cells already exist - they're on hand to refill again!). Not to mention the side effects of any drugs you're taking (which can be different for everyone remember).
It's so difficult I know, but the best advice I can give is to (whilst your relatively well, obviously) don't ever miss a meal - infact, if you can eat (smaller portions) five times a day, all the better. I've not missed a meal in about 6 to 8 weeks and I've lost 18lbs. I have literally eaten myself slimmer! The act of constant input of food has increased my metabolism so that I'm not automatically storing it. This will only go so far clearly, but what I have noticed is my portions have naturally gotten smaller. If you're like most people, you probably skip meals - but don't. Once you've got in the habit of eating routinely (it'll only take a couple of weeks), then start thinking about your food choices. It's hard, because what is commonly thought of as the 'healthy' diet - just isn't suitable for someone with Crohn's. So plan ahead, think of some meal ideas that you'll know you CAN eat and that you WILL eat. They must be balanced, they should always contain some fat, a palm size portion of protein and a hand sized portion of carbs. I couldn't touch salad and veg when I had my bowel, but I could eat a few things like if I skinned and deseeded a tomato, I could eat the flesh or skinned cucumber, mashed carrots and swedes - things which had no seeds or skins really. It's certainly not ideal I know, but it's these tiny little things that I can do that have made the difference to me. And forward planning is ultimately the key, working out in advance your weeks menu. Be good 80% of the time and the other 20% have what ever your treat is - mines to sit down and relax in front a good TV show with a rich, dark, gooey chocolate cookie and a cup of tea (tea - well, I am English). It's not going to be the perfect eating plan inlight of our food restrictions, but the weight will come off (just a little slower than normal).
Remember these principles:
Learn to eat routinely - never skip a meal
Forward planning - devise a weeks food menu that you know you will eat
Balanced meal should contain some fat, a palm size portion of protein (meat/fish), and hand size portion of carbs (always eat with this formula in mind)
Sit down for every meal, chew you're food well, put down your cutlery or food between bites
80/20 Rule - 80% well balanced meal choices - 20% your treat
Keep well hydrated
If you're as tired or as unwell as me - exercise is a no go - but if you can move a bit - instead of doing exercise (which is just hell to me - even when I'm well), do something that doesn't seem like exercise. Such as cleaning out those kitchen cupboards, or really cleaning the bathroom, sorting out your wardrobe. Not only does it use up calories - the sense of achievement and organisation feels good too.
Once you're back to much better health, you can start thinking of more suitable exercises to get the heart rate going etc. And you might be able to widen your choice of veggies and complex carb options, but don't be hard on yourself now. Just do the little things at first, that you know you can do and live with now. The principles I've given you aren't about a diet plan, they are infact, the way 'everyone' should eat - so there's no reverting back to a so called 'normal' eating diet. It's really working for me, slowing and surely it's happening. So the best of luck hon, keep in touch and let be know how you get on. If you want some meals ideas on the 'fat/protein/carb' forumula just ask. I should be starting Remicade (Infliximab in UK) very shortly too - so we can compare experiences!!
All the very best of health to you (hope this helped a bit).