Well, it's hard to say. You are indicating this is out of character? Puppies are usually bounding with energy and since this is a change, you need to pay attention to if there are any other symptoms going on. There are common things that puppies can get disease wise. Lethargy like you describe is one of the symptoms of many of them but there will likely be other signs. Parvo virus is a common puppy illness. There's intestinal and cardiac parvo (2 types) with dogs at greatest risk being under 6 months. This is a virus. Dogs will often vomit as well with it. And sometimes they get diarrhea. They are often dehydrated. There is also distemper. And worms. There's other viruses and things as well. That your dog is still eating is a good sign though.
Big question . . . are you immunizing your dog? Do you have a vet? It's time to get your pup checked out. Let me know what they say!