i am in a similar situation...this is my 2nd pregnancy & i am almost 30 weeks. last pregnancy, i gained 50 pounds (i was very small to start out with), lost all but 5 pounds after the pregnancy & now i have only gained about 19 pounds & it seems to be mostly baby (well, my bottom is a bit larger, of course!). i have been very nauseous still, with vomiting at least once a week. i don't feel like eating b/c food makes me even more nauseous, but i still do (i want a healthy baby!). at my last appointment, i had only gained 2 pounds in almost 6 weeks. the OB wasn't concerned at all, as long as i wasn't losing weight. but i still think something is off. i am measuring smaller, also, which isn't normal for me.
i don't think my response is going to help you, except to say that this is also happening to me & the OB said it was okay... so maybe it is? i don't know! i hope it helps to know others are in the same situation...
-H.