Female ,20, non smoker, never drink alcohol, and i always weighed about 90/85lbs. i've had a healthy pregnancy and delivery. how can i gain weight!?

Weight gain: The key to gaining weight is increasing caloric intake. You will need to take in 500-1000 more calories (than what you normally eat) per day to gain on the average one pound (lb) per week. Choose food that is dense in calories and gradually increase your intake. also, you will need to take around one gram/lb of your wright of protein to gain muscle mass. One egg provides 6-7 grams of protein

More calories: You gain weight by taking more calories in than you burn. Count your daily calories and eat 500 calories more each day than your usual calories that you have been taking in. At the end of one week you will gain one pound. Keep going like this until you reach your target weight which can be found in the height - weight charts in the web .