The same thing happened to me 2 (or was it 3) sessions ago. I have just started working on some...terrifying...stuff from early childhood, and in this session it took all my effort just to stay 'in the room'...I wouldn't have physically left but I could feel myself blanking out. In the end my t ended the session early to allow me space to 'find' myself (and she probably grew frustrated at practically no response from me for most of the time I was there)! I think that it IS very neccessary if whatever you are facing is too overwhelming, as long as you are safe and with someone. I know for me if I had been alone I would have been 99.9% likely to SI and be unaware of it until I became aware of the pain. My t has not seen me doing this before- she has seen me distancing myself but never to this degree. It was totally out of my hands and I was exhausted afterwards from the effort of trying to stay in the session. Just try to ensure that you are 'back' before you physically leave, for your own safety.
(((((freewill))))) good luck and I hope you can work through this ok.
irish