How to gain weight? I really need help it's getting scary!?

I have crohns disease and I'm losing weight like crazy. I used to weigh 130 and I'm 5'9 so that's good I looked super skinny at that weight and now I weigh 115 and everyone is making comments and I don't know how to gain weight. I need to know what foods to eat and unhealthy food hurts my stomach alot less I can't eat much things with high fiber. I can't eat nuts, granola, and many veggies and citrus I just need to gain weight :(