As long as you are eating and your baby is growing you are fine. You only need an additional 600 calories for baby. I have been drinking Ensure to get in extra protein and to help with weight gain. Talk to your ob about it at your next appointment.
I too have lost weight during this pregnancy. I am bigger then you so my dr isn't to worried. But if your worried contact your OB. Mine said as long as the baby is growing then it's fine. Maybe is bc you have been eating healthy food that you normally wouldn't have been eating while not pregnant. I wouldn't stress to much over it. But do contact your OB.