I understand that you are concerned about your puppy's relative lethargy compared to usual.
The positive news is that she is eating and drinking and is not vomiting.
Puppies go through growth spurts and may sleep more during that time or even have some bone pain which can cause them to be less active. This is more common in large or giant breed dogs.
It's also possible that she got into something that you aren't aware of and has some abdominal discomfort. Not enough to change her appetite, but enough to make her feel less playful.
Does she have a fever (rectal temperature greater than 103F)?
Does she have a runny nose, cough or loose stools?
At this point if her only symptom is relative lethargy I would give her time and see if she comes along on her own.
If she stops eating, starts vomiting, runs a fever or has a nasal or eye discharge then it is time to see her veterinarian.
Please let me know if you have any further questions.