why? no-one expects their cats to get pregnant if they are well cared for. If she is pregnant, then that is unfortunate, but not much you can do about it now. Suggest that you get some dietary advice from the vet though as, being still growing herself, she will need a dietary supplement to keep her own bone-strength up while she is growing the kittens.
Have your vet check her out, it may just be puberty!!