I experienced the same thing in my first pregnancy, and it seems to be happening again. I'm also at the lower end of the healthy range when not pregnant. I'm not a health nut, but I do exercise regularly and watch what I eat.
This is pretty common in first tri and not something to worry about as long as.you are getting enough water. The baby is tiny and will take what it needs. Especially if you're sick, just focus on getting enough liquids, eating what you can, and taking a vitamin. In my first pregnancy, I lost throughout first tri, made it back to my starting point near end of second, gained up until 37 weeks, and then started losing again (only a little) until I delivered at 39 weeks. Baby was smushing my stomach to the point that I felt completely full all the time.
My doctor wasn't concerned since my baby always measured a healthy size and it wasn't like I was trying to be on a diet. I tried to focus on eating as much healthy stuff (yogurt, other proteins, fruits, veggies) as I could early on so I wasn't wasting what little appetite I had on chips or other junk food. It was kind of fun to go to town during the month or so of second tri after my appetite came back and before baby got so big that he was taking up valuable cheeseburger space!
