they are worrying you over nothing!
mine is small too -compared to many others- but BIG for my doing!
i had a low bmi to start with as well.
my baby's measurements were just fine, lenght right on schedule but weight wise few weeks behind, they aren't worried as i'm a tiny person myself
as long as i feel it move around etc, i'm not going to worry about it, at all!
(((big hugs)))