They say long has your not gaining alot of weight in a short amount of time your fine
I lost weight to the doctor didn't say anything was wrong of course they actually know your going to loose weight in the first trimester due to morning sickness
Just make sure your doctor is aware of this
I was the same way until about 8-9 weeks then I started throwing up daily until 15 weeks or so. Its normal, as long as your doctor isnt concerned you should be fine