Weight gain ?

Has anyone here has difficulty gaining weight ? I'm 19w 1d and I'm consistently loosing weight. My doctor is not impressed but I don't know what else to do.

During my first trimester I couldn't eat and was throwing up constantly, now I'm eating all day, no throwing up, and eating fairly healthy I might add, but I just can't seem to gain any weight. I had my anatomy scan exactly 1 week ago and my baby is measuring 1-3 days ahead and is perfectly healthy, I don't understand, and I'm really not sure what to do now.