Is my dog growing too fast?

This is a question for your breeder. Note though that registration is only an indicator that the dog is what it is represented to be, and not a guarantee of "quality". Were you able to see her parents, and the other pups in the litter? Are they the proper size for the breed? Some breeders do falsify breeding information, it's called hung papers. But start with questioning the breeder, as that does seem awfully tall for such a young pup when her adult height is supposed to be only 14-16 inches. Note though that not every pup will meet breed standards, and if she was not sold to you as show or breeding quality, that may be the reason. Our last working line GSD pup was given to us for two reasons (bought several from this breeder). One was she didn't have the drive to compete in Schutzhund, and two, her size was much larger than it should have been, making her potentially too clumsy to compete in agility. She was, however, the best dog we ever had. Yours may just be a dog like her.

Ask whoever bred your puppy. Not every puppy in a litter will conform to the Breed Standard but that doesn't mean they won't make excellent pets. And puppies develop at different rates. She may have had a growth spurt and you'll find she doesn't get significantly bigger, for a while. Different 'bits' grow, while other bits don't!! With medium to big breeds, I tend to look at the frame being there by around 7 months, but it taking some time beyond that, up to 2 years for males often, before the pup 'bodies up'.

KC (which?) registered only means her parents were KC registered so her litter was able to be registered - nothing else, much as good breeders should be trying to breed to the Standard.

KC? What do you mean by that? If it's AKC or CKC - Canadian Kennel Club - then you have a pedigreed dog. If it's UKC or another bogus registry then you have a mutt.

The size your dog should grow to depends on how big its parents are, whether the animals are pedigreed and whether the breeder is breeding to the standard. Did you even see the parents of this puppy?

If she appears to be a lot bigger than the breed standard, you can request a DNA test from her and both parents at your expense.

If she turns out to be a crossbreed, then the breeder owes you a refund on the dog. If you are at all unsure, ask the Citizens Advice to put you in contact with Trading Standards UK.

Contact the breeder that provided the papers and ask them and/or ask the vet that provided the wellness exam and did the fresh fecal specimen within 48 hours after getting her.