I'm assuming you've already read the info on this site and have started (or are planning to start) Heather's diet. If so, you're on the right track! Once you're stable and eating regularly, you'll gain the weight back; just know that it may take a while. For some people, it's hard to lose the weight, for others it's hard to gain it back.
Oh, and please know that you're not alone! I was diagnosed with IBS-D when I was 18 and in college. That first year I went from a healthy 130 pounds to a puny 105 pounds. At 5'6" I was so underweight I couldn't even donate blood. For me, the weight loss took an emotional toll. I had never been so small before, never felt so weak and low in energy. It was as though I were a different person! I didn't recognize my own body! I mean, I hadn't weighed 105 pounds since 6th grade!
Eventually, I went back up to 124 pounds and felt great! Unfortunately, after being stable for a few years, I had some sort of stomach flu or gastroenteritis last fall (which triggered the IBS again) and now I'm back down to 108 pounds. But I'm doing better and I have gained three pounds since October. I'm just following this diet, lifting free weights to get the muscle back, and taking care of myself the best I can. I also try not to focus too much on what the scale says, but more on how I feel. Here's a link about gaining weight on the IBS diet.
I'm sorry you're suffering from other health conditions as well. If your doctors are not going to listen to you and they want to assume you have an eating disorder, I say, "Get new doctors!"