I used to have an under eating problem too, with my ribs showing and stuff. I thought being skinny was muh guhnetics because I would eat fast food and junk. After counting calories, you realize even though your eating shit, you're still not eating enough calories. You need to plan your meals and eat them, calorie dense foods with lots of protein. You need to stick to an eating plan like a fatty needs to avoid food. edit: Also I ran a ton too when I was too skinny. Decrease cardio a little (don't get rid of it) and lift weights more.