I've had a similar experience. It happened at my old T's office. Just got so so tried, just like I was watching everything but was so tired I couldn't respond. I actually ended up laying on the floor and the T was slapping the floor beside me and clapping hands. I wanted to say something, like I'm here, I hear you. But I just couldn't physically do anything but lay there.
I'm like you I'm not really sure if this was a dissociative state or just being so emotionally drained from the therapy session. I was there and knew what was happening around me, but so drained of any physical energy to respond. And trying to put a sentence together was just too much to do at the time. Kind of like your brain and body have to reboot. Shuts down and then takes awhile to come back up to full power.
I don't have an answer to what this exactly is. But I do understand what your talking about. I still get really tired after T, that I'll go in lay in my car and rest for 30 mins or so till I'm able to drive.
Do you think that this was maybe some form of co-consciousness? Since you felt like you were 5.