Losing weight

I had an appointment with my obgyn today and apparently I lost 2 lbs. I know it isn't much but my doctor said with me being underweight already it isn't something he likes to see. I've honestly been eating gross amounts of food and have no idea how I've lost any weight. The doctors explanation was "the baby's stealing all your food". I'm not really sure if I could even handle more food than I'm already eating. He said he wants to see me gain 5 lbs by my next appointment. Wish me luck :/