How old is she, do you think? Your vet should have been able to tell fairly accurately by her teeth. Little bitty puppies do sleep a lot...and some pups are just not very reactive. One of mine was so mellow as a pup people joked that I must be giving him Valium...! Mind you if she's that tall, she's probably a few months old.
Does she get curious and chase things if you throw them, like a ball or toys? Does she react to louder noises, louder than clicking your fingers? Does she play? Could she be uncomfortable because she's teething, or just shy and uncertain because who knows what sort of start she had in life...?
Just throwing out some notions. If she's real listless, and/or doeasn't eat well, I'd ask the vet to do a blood panel on her or something to check for underlying problems...heart, thyroid, like that.
Or, she could just be a very serious, quiet pup! I sort of like dogs like that.