As long as you are eatting & taking your vitamins, you should be just fine! I was in the same boat. No sickness & I lost weight in the beginning and started gaining when i want to my 16 week appt. Im 21+3 now and have only gained 4 or 5 lbs. My doc says Im doing great & so is baby!
As long as you're eating enough food everyday, and not losing like massive weight, you should be fine. Some women don't gain any weight through their pregnancy. I mean obviously this is not the time to diet or anything, but like I said as long as you're eating enough you're fine.