My 1.5 year old dog bit my mom today. It was totally unprovoked. My dog has a problem of resource guarding. We worked on it with a behaviorist for a few months and it had reduced a lot. He doesn't like being told to drop things but usually does it after a while and everything goes back to normal within a minute or two. He doesn't even pick up random things anymore. We were taught how to look for all the subtle signs that I dog can give off when its uncomfortable. While the resource guarding never got to 0, what really disturbed me is that today there were no signs of him snapping.

We haven't had an issue for almost half a year. We haven't even had teeth bearing in months. I thought all this was behind us. Today my dog was biting a mattress that we keep on the sofa. So my mom asked him to get down, which he did. A few minutes later, he was continuing his sleep on the floor a few feet away from the sofa when my mom simply just went to put the mattress back in place. He suddenly bit her on the arm.

I am so worried. The last few months have been so great after a rocky start with him. He is the sweetest dog. Why would he behave like this? What should I do next? He seemed sad after he did it. My mom closed the room of her door to get some sleep and he slept outside the door constantly. How should I behave with him next?