Hi Lisa, I have been in the same boat. I'm 5'5 and my lowest weight was 100 which is bad for me. My Aunt was watching me get dressed one day and she pulled out a "Ill be glad when you gain some damn more than anything but its hard when your sick half of the month and scared to eat, and people who have problem with too much wieght just cant understand it.
I started gaining weight when I got happier and went on with my life. I am now at about 113-116, and I"m so glad I got my shape back. I started making myself eat a couple of hours after attacks. I was losing weight because I had a sincere fear of food. And some days I still only eat twice a day. but the days I feel good, I stick it to it. I eat big snack in between meals, and that gives me leway ont he days I dont feel good. Keep peanut butter and crakers. I think my ability to eat eggs helped because I ate omelets at least 3 times a week. Try something high in protein. I found when I stopped worrying about gaining weight it came, also to be honest when I became more sexual (which I had ceased due to my IBS depression) I started gaining weight too.
I still have problem finding vitamins though because my IBS D comes with those, but I"m anemic and need them.
Hope it Helps But one thing I have ralized about IBS is that everyone's case is different and what worked for me might not work for you and vica versa.