Jada was doing this very same thing for the last few days. No fleas, no skin irritation or anything I could see. I took her in yesterday and it was her anal glands. She had never had any problems before and never had them expressed. Both glands were full and one was stopped up. My vet was able to express both sides and it was awful at the amount of stuff that came out
It may not be anal glands as your puppy is still young, your vet will most likely test for parasites as well.