Your situation is nothing like mine but I thought perhaps, it might ease your mind.
I am 28 weeks pregnant... I have NOT gained even one pound. Rather I have had chronic diarrhea (which they can not figure out why) and I lost 10 pounds, thankfully I just gained the rest of that back.
All the while, I "look" small from the outside but when they measure me I measure 1-2 weeks ahead! I wonder how can that be? I look tiny and haven't gained a thing.
I too had a Level II ultrasound and "he" looked perfect. All growth was on target.
My Doctor had to remind me that our bodies are "protectors" and we will go through hell first before it affects the baby. He also told me that the position of the baby and how my uterus has stretched plays a big role in how I measure. Basically he said, on the inside, I am "All Baby."
Obviously your doctor knows how many weeks you are, and I believe if he/she was concerned about the differential something would have been said. As for the lack of weight gain, that too, if they were concerned they would have said something.
My advice, relax and monitor your activity - don't do too much (easier said than done) and see what the next OB appointment brings.
Good Luck to you and that baby! :)