How can I gain weight eating healthy foods?

My husband left me a month ago. I am doing quite well aside from losing about 10 pounds which I cannot afford. I was already a small size 8 (UK terms) my clothing is quite loose on me now. People are beginning to comment how thin I look. I am 5' 2" and weigh around 47 kgs. The first place the inches disappear is around my waist and thighs. I cannot eat fatty foods or much dairy because I have IBS and get ill from this sort of thing and always have done. does anybody have an ideas as to what I need to do to put that weight back on and keep it on?