go buy whey protein (Optimum Nutrition) and some rolled oats... mix 1.5 scoops of the Whey with 1 cup rolled oats, add 1-2tsp sugar and milk or water.

eat this a couple times per day, also eat regular food... lots of lean chicken and LOTS of complex carbs if you're really that small... spread meals out as much as you can (5-6 times per day eating). just because you're not hungry doesn't mean your body doesn't need energy