This happens to me too. I will be chatting away and suddenly become still and withdrawn. I will put my head down and avoid eye contact. In the last few months t has started to say "it's happening again Mona" because sometimes I don't know when it's happening.
T will say what just happened because your mood changed so rapidly. I feel like I can't talk when this happens and t says " I can't support you if you disconnect from me so try to stay present". She asks me to put my feet on the ground or to look around the room. This doesn't always work and t gets very frustrated. She will keep repeating my name until I look at her.
This week t asked what happened right before my mood changed I wasn't really sure. I think it's when I feel judged or shamed by something she says but I told her that and she just said I was blaming her and that something happened in me that I chose to withdraw.
She says she feels sad that I do this to myself. I often say I can't speak and she says can't or won't because you are doing this to yourself.
She has a counter transference reaction and feels as though I am with holding and with drawing for her deliberately and she gets frustrated. When she gets annoyed at me I become worse and stay quiet for longer. I have told her that I don't like it when she gets angry, it scares me but she says that's making excuses because something happened first to withdraw.
I am sorry about my rant about my own therapy experience but I was surprised to see how other ts reacted. You have a good relationship with your t retro perhaps there is something happening within the relationship that is causing this block! I hope you can try explore it together in a kind supportive way to you.