You should talk to your doctor.
as long as the fetus is growing and developing normally you dont need to worry too much
You shouldn't be losing weight while pregnant. You should get checked out for both hyperthyroidism and gestational diabetes.
Don't worry about it. The baby has first dibs on the nutrients, that'll be why your loosing weight, it'll be big and healthy, and leave you with the left over nutrients it doesn't want.
I was normal weight before pregnancy,
Over the first few months of my pregnancy i lost 25lbs.
My daughter was born weighing 9lbs 3oz.
... and don't try to force yourself to eat too much, in the last couple of month everyone piles on the pound whether on not they want to.
If you are still worried, the midwife/specialist can measure the size of the bump to check the progress.
I wish you all the best. :-)