New "Puppy"

For the past several days, Collin has crawled around on the floor and barked like a puppy. When I talk to him or ask him questions, he replies with "ruff, ruff". He likes for me to scratch behind his ears or his head when he is in "puppy mode". I am not sure what started all of this because he will not talk during this time. If I ask him when he is Collin again, he will revert back to being a puppy. I guess this is him using his imagination.