Depending on how old your puppy is, it may just be sulky and missing its mother. You need to keep an eye on this though and if it doesn't become more energized in a week or so, I suggest taking it to a vet for a check up.
Why Has My Puppy Got No Energy?
It might be teething and have toothache.Generally when puppies are teething they are irritable and disobedient. It's a normal stage in your puppy's life and it shouldn't last too long, but you might want to take it to the vet just to be sure.
