My puppy is realllyyy lethargy, what is wrong?

He could be having a vaccine reaction. or he could have something going on that is not related to his vaccination at all. the only way to know for sure and to help him is to bring him to the vet. If you are concerned about him you need to bring him in...no one on here is going to be able to tell you what's wrong with him. puppies can get dehydrated very easily...a few hours of vomiting and diarrhea will make them very weak and lethargic. you need to bring him into a veterinarian immediately. you do not need to spend 4-6 hundred dollars - let the dr. know your limitations financially from the beginning and make sure if you go to an emergency clinic that you let them know exactly when he had his vaccinations and what medications he has been given. Please bring your puppy to the dr.