Hi UV,
I experienced this fairly pronounced one time in session. I have no idea what triggered it, as it seemed to come out of the blue in session (I'm sure it didn't, but it seemed like it), but it left me all out of sorts for several hours. This was only in my 4th or 5th session with my T, so that also added to the peculiarity of it. Anyway, I felt like an observer in my own body, kind of like I was the outline of my figure sitting in the couch, but still able to see things through my own eyes. Very odd.
After that session, I began to tell my T when I was feeling that way again (or even just beginning to feel that way) so that I didn't have to leave feeling so disconcerted. I know you said your T noticed something was up, but did you tell him anything about what you were experiencing?