I had this LOTS of times. Especially when we talk about a certain topic that I just can't seem to get past. I blank out, I dissociate, I am not present I have total memory loss.
My T is usually really good at telling me what happened. She said I get that "look" on my face and she can't tell if I am listening or if I am miles away. Even if I talk, my mind wanders off. My T knows this is a form of dissociation. When I am restless about it, I ask her and she tells me quite detailed what happened or what we talked about. She says: "It always happens when we scratch the surface of X" but she still brings it up whenever she thinks it's appropriate.