How old was she when the pet store got her? if she is 4 1/2 months now, was she only 6 wks?… (if she was there 3 months?)... And does this pup have papers?... Just wondering.... just off hand, did the Vet agree with her age of 4 1/2 months? It has been known that pups sent to pet stores are not the age they are claimed to be... easy enough with pups to know the age with baby tooth loss.... Also, it might be a good idea to have her eyes checked by a board certified Ophth (your vet can recommend one) just to make sure there is really nothing going on with her eyes that you should know about....
I agree, if 4.5 months, 12 lbs is not that small....
Good luck... with her...