Gaining/losing weight.

Okay, my pre-pregnancy weight was 167. I was in the process of working out & trying to get healthy when I got pregnant! (it was planned) but anyway, I had horrible morning sickness in the 1st and early 2nd trimester so I was dehydrated a lot and lost a lot of weight. At one point I was down to 141. I just went in for my 29 week app, and I still haven't gained ANY weight, I only weigh 149. They aren't worried at all because my son is gaining plenty and doing just fine. But it's making me nervous. I eat just fine and drink just fine, I don't know what else I can do to gain it.