I have the most passive shih tzu. Sometimes when he plays with Remy and she is telling him off in dog language I will see his lip twitch. It is the funniest thing.. then he will growl and jump at her and they go round and round. If you could hear them play you would think they are killing each other.
When we walk there is a chi that runs out of its house and "attacks" my guys.. they tuck tail and run between my legs. My two boys are bigger and I often wonder why they don't stand up to it. One of my guys stood up to a large lab to protect me, so I get confused.
So you little fur but is either playing, or not happy with the bt- I think we know which
Good luck and have fun