How do i gain weight without getting too chubby? (in muscle)

im 16 and im 107ish pounds and i used to be like 112 but i exercise like crazy, i do karate, kickboxing and tennis almost everyday and im a very picky eater. im almost a vegitarian (sp) i only eat chicken for meat and my doctor said that i shouldnt be losing weight. i just want to bulk up a little just to be healthy. i'm still going to be exercising and my metabolism is prety high. what should i do? protein shakes?