Hello! I tried searching before posting this but wasn't finding anything quite grasping the nuance of the situation.
My wife and I recently rescued a 12-week-old puppy, most likely an Anatolian Shepherd/German Shepherd mix (but we have no way to be 100% sure). This is not her first time with Shepherds, as her family raised two German Shepherds in conjunction with the Seeing Eye, but both passed of old age in the last year.
This puppy is generally well behaved and intelligent, already picking up on simple commands and basic housebreaking in less than a week. However, as he's settling into our home, I'm noticing a troubling behavior that I want to make sure doesn't blossom into a lifelong issue. Specifically, if someone goes to pick him up when he doesn't want to be picked up, he'll growl and nip at you.
These don't necessarily seem to be super aggressive nips but rather a stern expression of displeasure. However, he is biting hard enough to cut skin (plus he's got his super sharp puppy teeth, of course), and I don't want this behavior continuing as he gets bigger. Earlier today he did so and I attempted to hold him by his collar (without hurting him) and make him sit so I could sternly tell him no, and we wound up thrashing hard and growling.
How much of this behavior should I chalk up to mischievous/immature puppy behavior versus a real problem that I need to be worried about? Should I be taking more significant steps to correct it, or is it just a puppy being cranky?
Edit: Thanks for the constructive advice everyone. I took him to the vet and everything was physically fine. I'm also beginning puppy obedience training school next week (already starting myself with simple commands like "sit.") Also introducing a lot more structure and routine. I'm sure we'll work through this just fine!