Suddenly beginning to bark at me

My 5 month y/o English cocker spaniel mix has suddenly begun to bark way more often than before.

Everytime he used to bark (up until a couple of weeks ago) it was clearly because he was awake to long and very tired even though he was running around like a maniac

But for the last couple of weeks he has been barking at me (sometimes even showing teeth) maybe 1-2 times a day. Can this be a sign of agression (even though he loves absolutely everyone and everything). Can it be from the pain of growing new teeth? Is it a teenager thing (he’s definitely starting that face already now). Can it be something different?

Does anyone have experience with this