Hm, she is small, but you said you were taking her to the vet? Just be sure to discuss this with the vet.
It could be that she is a runt, a friend of mine owns a cat who is a runt. They are so small and sweet

And they do seem frighteningly small when they are that young.
It doesn't sound as if she is sick, if she runs and plays, and eats.
Again, discuss this with your vet, and make sure you keep track of how much she eats, and how much weight she gains.
Good luck!