3 month old puppy with resource agression? I"m confused.

Over the past month, I've noticed that my chorkie puppy is aggressive when he's got high-value treats, like pig ears. He'll generally allow his bowl to be picked up, and will even allow me to pick up his pig ear IF I do it from the front. He knows the word "trade" and will happily also let me pick up his pig ear to get another treat. If, however, I try to pet him while he's enjoying his pig ear, or if I even approach him from the back, he goes into guard mode. He's tried to bite my hand twice now when I've tried to pet him while he has the pig ear. It's very confusing because, as I said, he's fine with having the pig ear actually taken away from the front - I've worked on building his trust by teaching him the word "more" means that I add an additional treat (usually Kong stuffing) to the ear and then return it to him. So he's got no issue with that.

Not only is this a big deal because I don't want to raise an obnoxious, vicious, little monster dog, it's important because he's very mouthy guy generally so when we take him out to stores and such he's in a body carrier (like a baby) and I'm holding a chewie similar to the Kong. He's never been aggressive about that before. But I'm concerned that it could generalize. Especially since he snapped at me when I was holding him and his chewie while he chewed on it earlier today.