I know all about those feelings of discouragement...
Yeah I bodybuild, but due to the intensity and diet I use for it, I can't do it while I'm flaring like this. I got up to 215 at my highest (but still wasn't done), and had some trimming to get some fat off, and was in that process...then a few months later (got down to around 185 and kept most of my muscle) in fall of 2006, I was diagnosed, and put in the hospital. That flare lasted 7 months, but I was able to work out starting December 2006 because it calmed down enough, eventually, around Easter 2007 I got into my only remission since diagnosis, and got up to a good 185 and started a cut because I had plummeted to 160 during the flare and some new gains were fat...well, then August 2007 this current flare hit, and I dropped down to a bottom of 145 at the worst over the winter. I haven't been able to do more than a few workouts during this year long flare, mostly because of the severity and the prednisone I was on for most of it. I'm 5'11".
My diet now has to be for Crohn's sake, not bodybuilding (which I loathe this fact), but I do know there's a few items that work to hold onto whatever muscle I've got left, mainly lean proteins, lean red meat, egg whites, chicken/turkey breast, fish, hard cheeses....anything fairly low in sat fat (except the cheese, which I do during low carb times) and high protein salvages muscle. When not lifting, if you don't use it, you lose it. I drop major muscle when not working out and I assume that's why you did. 5'8" and 170 is decent and when you plummeted I'm sure you did feel skinny, because yeah, relative to your goals and your prior weight, it technically is thin. Normally I consumed when healthy, about 3500-4500 cals a day while bulking, and worked out somehow or another 6 days a week. Mostly I ate the above lean protein sources, and lots and lots of oatmeal with peanut butter in it, lean dairy, salads, veggies, lots of fruit, whole eggs, tuna, cereal, whole grain products, and tons of water.
It's odd being a bodybuilder in this world, everyone wants to drop weight, and honestly, yeah most people should, but nobody seems aware that even a few pounds of muscle will help them with that more than they realize. So it's also odd when I tell people I want to be 50 pounds heavier with around 8% bodyfat, instead of the 165 lbs I am now. (I want like The Rock or Vin Diesel) But I just remember to ease my mind, even with Crohns I will get to my goals, and probably before most people in the civilized world get to theirs, it's about perserverence.
You say you can work out now some, if you can find out from your doc if the condition and location of your Crohns would be harmed by Creatine, for the most part I believe it's okay, I did Creatine during my first flare to test it and had no worsening, and for 4.5 months of remission also, so I know it didn't cause me to flare, or make the flare worse, I got into remission while on it and kept it, so it must have been fine. Remember, your body makes creatine on it's own and the average American eats 1 gram of it a day in foods, so supplementing 5 grams/day shouldn't, in theory, be detrimental. For food, that's dependent on your Crohns, how bad it is, and your trigger foods. If dairy is fine with you, fat free dairy is great, or the foods in my second paragraph above. And also, lots and lots of carbs, the healthy kind, brown rice if you can handle it and oatmeal if you can handle it. If not, go for the refined grains. Healthy fats like olive oil and nuts (if you can handle them) also for your fats. I can't be too helpful in the diet area for you other than that as of now, and it's possible you already know this and more, these are just some basics, I just don't know your Crohns and trigger foods.
I can help you more if you want, you'd have to post some info on your Crohns and goals, and what kind of foods you can tolerate.
