I've been there although I was fortunate to have a big period of time in between my eating disorder and when IBD came along. From about ages 16-19, I dealt with anorexia (never bulimia - I am extremely averse to vomiting). At 16, I was so thin to begin with (122 lbs at 5'8" which was underweight already) but I had a bit of cellulite and my mother called me "disgusting", which horrified me to the point that I started starving myself and hating myself and feeling like I really was disgusting. I kept myself between 105 and 108 lbs - any more and I felt like I was a hippo and would starve myself for days, any less and I'd start to become noticeably skeletal and would get afraid that someone would notice and make me get professional help. At 19, I had a good friend who helped give me perspective and I started to heal from the damaging thoughts and behaviors around that time. Sadly, my friend suddenly passed away in a house fire when we were both 19, but thankfully that didn't impede my recovery (if anything, it made me want to get well to honor his memory).
IBD came into my life just before I turned 30, so there was about a decade in between the ED and IBD where I was reasonably physically & mentally healthy. I'm fortunately well enough in my thought patterns that I recognize now that too thin = sick and healthy weight = okay. If anything, my thought patterns are now basically reversed from when I had an eating disorder. The first year I had IBD, I was so so sick and miserable and was losing weight out of control. Looking back on photos of myself at that time, I can recognize that I was too thin and looked very ill (I got down to 115 lbs and my thighs looked like toothpicks). After that first year, I went on steroids and gained a bunch of weight but felt so good, that I've come to associate weight gain with health and weight loss with illness. I'm currently at about 145 lbs thanks to steroids and I'd like to lose about 10 lbs as my healthy weight is about 135. But, I'm okay - I'm not going to starve myself or do anything drastic to lose those 10 lbs. I can do it the healthy way, or I also recognize that I'd be just fine staying at this weight. I'm not overweight, I'm not fat, I'm not disgusting. I'm okay!
So, even if you do gain weight due to steroids or whatever other cause - please recognize that it's okay. If you're very ill and losing weight, you probably do need to gain a bit to get to a healthy weight. You're not gross, you're not fat, and it's okay to gain weight. And those models in magazines are photoshopped like crazy (my father-in-law works at a printing press and he regularly sees the pre-photoshopped pictures, he said there are stretch marks and blemishes and they slim people down considerably and do all sorts of crazy cosmetic fixes in photoshop - nobody looks like those models, not even the models themselves, I promise). Having flaws is okay, gaining weight is okay, being okay with yourself is important and totally possible.
It took me a long time to learn that.