I've been where you are! I'm also 5'6" and a year and a half ago I was down to 104 pounds and felt like I was dying. I felt horrible and gross and weak. Now I'm 120 pounds and I've been at that weight for six months now -- and I'm really happy body-wise. I get extra calories in by eating the safe quick breads and smoothies listed on the site. Luna Bars have also been a great snack for me -- easy on my gut and an extra 180 calories. I've been able to maintain a healthy weight without having to eat things that aggravate my IBS (no dairy, no red meat, no high fat foods). Fortunately, I've never been anemic or malnourished.
However, I should point out that in addition to IBS, I found out that I had lupus and being in a flare was causing me to lose weight. If you feel that you're eating well and getting enough calories in but still losing weight, you should see a doctor -- just to make sure everything is okay.