I'm 20 years old and I'm very short about 4'10" but I only weight 81 lbs. I know that's not enough for my age and height but I don't know how much I should gain. I've always been small and in the past two years have had health issues that caused problems for me to gain weight. My doctor says just "gain some and we'll see" but I would rather have a definite number. Any advice?