A.E.
My daughter was 18 lbs at her 18 month check-up, and my doctor diagnosed her as being PETITE! I have 2 daughters, 3 1/2 & 2 (as of 9-7) and they are almost exactly the same size - seriously both wearing a 3T. Sometimes my younger daughter fits into the clothes that are too big for her older sister! Other than being petite, my 3 year old is right on track.... actually I think she is super smart, but I may be a little biased! :)
Children all grow differently, so if the doctor is doing all this testing because she is petite (with no other problems) I would really get a second opinion. Don't worry mom - your baby girl is probably just going to have to buy petite clothes & hem her pants for the rest of her life like the rest of us shorties have to. Call another doctor just to be sure, but don't worry until you have to!