That seems like a puppy acting like a puppy. Like little kids they have a tendency to see what you will and will not tolerate. And running back and biting is just a leftover habit from romping with littermates. However, i would put a stop to the biting asap, even mouthing IS NOT allowed lol. What i have found works best is grabbing the puppys lips in a mock "bite" not hard or anything, then i look directly in the eyes and firmly say "No". I've broken every puppy i've ever had this way in like 2 weeks. But every dog is different so .. i wish you luck! And enjoy your pup!!!