new rescue hit a wall at week two - he was fine and now he's not fine, is this normal

We adopted Ruger, a 3 year old hound mix, two weeks ago. Week one was honestly amazing - he was calm, curious, followed me around the house, slept through the night. I thought we got a unicorn rescue dog. I texted my sister saying it was way easier than expected.

Week two is... different. He's started barking at things outside that didn't bother him before. He had two accidents in the house after a week of zero accidents. He started resource guarding his food bowl, which he never did at first. And he's been way more restless at night. I keep second-guessing everything, like did i do something wrong, is he traumatized, did something happen i'm not aware of.

The rescue coordinator mentioned the "3-3-3 rule" but didn't really explain what to expect in terms of specific behavior changes. Anyone who's been through this - is week two behavior regression normal? And is there anything specific i should do or not do right now?