Sorry to hear about your health and weight issues. That's so frustrating, isn't it? Because everyone thinks you should be happy for being thin.
The reason I had lost so much weight is because I had this awful sinus infection and the docs had me take sudafed round the clock. I was so thirsty and jittery from the sudafed it made it nearly impossible to eat. (the active ingredient is pseudoephedrine, derived from the herb ephedra, which people often use--possibly unsafely--as a weight loss drug.) THey told me to cut out all dairy which also altered my diet. On top of that I was trying to lower my carb intake (especially simple carbs) to avoid a yeast infection, which I thought was inevitable since I was on antibiotics for a month. I also took probiotics, and luckily, I ended up with no yeast problems except a thin tongue coating which was easily taken care of by probiotic powder.
I really didn't like how everyone was constantly saying to me, "you're sooo thin" and "you've lost sooo much weight" and I was starting to be really concerned because I was not wanting to put my body through the stress of weight loss at this time. I went from 125 down to 110. I'm 5'4".
A couple years ago after a quite unpleasant surgery (to repair major damage due to an unwanted episiotomy). I was on a liquid diet for a time, and then afraid to eat much because what goes in must come out, so I went down to under 100. That was really scary. I thought I looked like a skeleton.
When trying to gain weight back, I would just try to eat larger portions at mealtimes and make sure to keep good snacks handy. Like I said, lots and lots of hummus. I am now about 115 and feel just right.
I wonder if looking in a good weight loss book for the foods they advise you to avoid might be a good start. (As long as they are reasonably healthful.) Fats pack calories, too, but you want to choose something good for you like olive oil. Potatoes fried in olive oil. Yum... I will try to come up with more ideas...
I have not had any problems with my children's weight, but if I had concerns about them being underweight, I would start by offering snacks often and presenting them in an appealing way. It's all about marketing with them!
Is it the dietary restriction that is causing you problems or just the damage to your digestive tract?