Puppies - I lost a few kg's with DD1 & then lost about 10 kgs with DD2 in the first trimester. Both times I was on the under weight side anyway. I put on 11kgs all up by the end with DD1 & 4 kg's from start weight with DD2, but I also put back on the 10kgs I lost.
Baby is taking everything out of you. No matter what, baby will take it's nutrients first, then lets us have what ever is left over.
Your bubs is fine, don't stress.
With DD2 I was so sick I pretty much ate nothing for 4 months. What I did eat didn't stay down for very long & she was my biggest baby at 8lb 9oz.