The vet should be able to assess her body condition and tell you if she is too thin. Could just be her physique, but definitely have her checked. Stool should be checked annually.
I can feel Maximo's spine easily, especially when he is lying flat out on the floor, and yet he could stand to lose a few ounces.
Teddy was a boney waif for the first couple of years. He has finally started filling out a bit. He eats the same amount of food that his bigger brother eats, but he is very active and I think has a super-fast metabolism.