Eat more, duh. But eat healthy, increase your protein intake(1/2 gram x body weight for women is often recommend... ie. If you weigh 100 lbs, you need 50 grams / day at minimum). Also continue with your weight training, heavy weights( 65% to 85%) of your 1RM max for 3-5 sets of 6-12 reps is recommended for muscle hypertrophy. And lastly...time.