We just got a Jack Russel female dog from the shelter a couple of months ago. She is 5 years old. One of the annoying behaviours she has is to become aggressive all of a sudden and even bite sometimes.
This happen often in relaxed scenarios where she is resting/sleeping next to us on the couch and being pet very gently. After some minutes she might just start showing her teeth and even biting without any particular reason.
She does it more with my girlfriend and she bit only her so far. She does make noise when it happens with me.
I can't understand what happens to her nor how to stop it.
P.S. she likes to pull on ropes and towels and would come ask us to pull it and she seems to enjoy it but we avoid it because she becomes more aggressive. But she doesn't bite at all when she does that. It's clear it's a game for her.