Its difficult to lose but relatively easier to gain weight, so you need not worry much. Take good sleep and good diet that s all you have to do and stay happy as well. You must rule out hyperthyroidism, tuberculosis, cancer, malabsorption due to any kind of prolonged illness, any eating disorder. So if you are not having any of the above mentioned problems, its not difficult to increase weight. Avoid prolonged meal time intake and balance your energy intake with the amount of physical activity. Take peanut butter, Egg, Banana, good amount of protein if you are exercising as well, so it has to be healthy weight gain and not binge eating.....tc care and go ahead happily