I'm in the same predicament! Nothing short of 1200 cals a day will get my body to drop below 143 (I'm 5'6.5"). Anything over 2000 = fast weight gain. I run 25 mpw, ride 150, swim 30 minutes 2x a week, 1 hour martial arts...etc, but no dice on the weight loss.

Have you tried fitday? It really prevents me from gaining weight, and I find knowing sodium intake and fiber really help for training purposes. Since you have Crohn's it might be even more useful. I don't know that much about Crohn's, but it seems that it might have a lot to do with weight loss.

If you gained weight while riding you may be drinking too much. Hyponitrimia is a hot topic in the running community right now, any weight gain during excercise is generally bad. I can't find the article now, but regular gatorade is too low in sodium, so it is possible to become hyponitrimic from gatorade (though harder than on plain water). Here's a little blurb

http://my.webmd.com/content/article/36/1676_50497.htm

Also, excercise over 2 hours triggers cortisol production, cortisol is a stress hormone which causes water retention among other things, so if you noticed the gain the next morning, that's likely the culprit. I only ate 1200 calories the day I ran the Richmond marathon (fell asleep), but I was up 3 pounds the next morning (marathon burned about 2600 calories). I'm always up a couple pounds the day after a long run or ride.