Definitely sounds like dissociation. It's a defense against pain or knowledge that you're unconscious self feels threatened by, or a defensive response to past pain triggered by some connection to that pain. The trigger could be a word, a sound, a sight, a thought or smell.

You really, really need to tell your T that this is happening. That you've noticed it in response to a specific word is terrific! This will allow your T to be alert for when this happens. You may or may not exhibit any signs. And it can happen quickly or slowly, which makes it difficult for you to notice or intervene by yourself. There's nothing to be ashamed about! It's an incredibly functional defense that has done its job very efficiently in the past; now the challenge is to get beyond/around it because you have other opportunities to deal with the pain that will make dissociation unnecessary. Your T needs to know because it will impact her decisions about how to best conduct therapy.

It's especially important to tell your T before you begin any group work. Many Ts feel that dissociative responses can be triggered excessively in group situations and that group work may not be in your best interests, depending upon the focus of the group.

I used to dissociate almost every session when I started therapy. I would become unnaturally still and be unable to speak. I also couldn't really understand what my T was saying, though I was aware of him speaking. It was like being under water. My vision became unfocussed, and I experienced spatial disturbances, like being "behind myself," often at a different angle from my T. My heart rate would rise, my breathing become very shallow, and my T told me my face would take on a mask-like look. The need lessened in proportion to the trauma processed during therapy. Now, it hasn't happened in many years, even in the face of triggers that would have prompted it in the past.

You are not defective in any way. Your experience is extremely common, your T should be familiar with it, and she will only respect you for bringing this awareness to her attention.