Maybe you are not eating enough food? How is your diet.. try and maintain 1200 calories a day plenty of water.

Working out you might be building muscle mass.. are you eating enough protein?

MUSCLE WEIGHS THE SAME AS FAT.. 5lbs of fat and 5lbs of muscle is still 5 lbs.. .. one is more dence then the other. but the actual weight is the same.

Try measuring your BMI rather then lbs