Yes, I had a very similar experience a couple of months ago - I started this thread about it:
https://www.myptsd.com/threads/feeling-mortified-after-yesterdays-session.56932/
In the above thread, I mostly talked about how dissociated I was but it was more than that. I don't think it was a flashback, because I don't think I have those. But my therapist has since described it as a triggered trauma response, which was then kind of mixed in with dissociation. I think there was both re-traumatisation and severe dissociation - I think I kind of swung wildly between the two for about 2.5 hours!
I ended up standing in the corner of her room, backed up against the wall having reversed over (and nearly fallen over) a fan on the floor to get there. And I was basically just there frozen in fear for I'm not sure how long...maybe about 20 mins...while my therapist tried to talk me down and ground me. I couldn't interact with her. I couldn't move. I just felt a real sense of terror but I didn't know why.
Then when I finally moved I ended up facing another wall – basically my forehead and toes nudged against the wall and the palms of my hands flat against the wall. And I just stood and my entire body shook for about 15 mins.
When I came out of it, I was completely disorientated, could barely stand up, had no idea what had been going on, felt totally mortified.
It was horrible. I felt mortified. The fallout was awful – I pretty much wrote off a week because I felt so terrible. I couldn't shift that all-encompassing sense of fear for days and days even though I still couldn't really work out where it had come from and what had provoked it in that session.
It took me weeks to recalibrate from it – in fact, it only really feels like I'm pretty much back to normal now. So the impact was huge.
My therapist was lovely, totally supportive and really went above and beyond (I was there an hour longer than my session time and she bought me a chocolate brownie to try to get my blood sugar levels back up so that I could get more present and safe to travel home. And then she checked in with me pretty much every day that week). Didn't stop me feeling embarrassed and ashamed though. We've largely left it alone since as she's been deliberately keeping things light and gentle in sessions so as not to trigger anything to that extent again. But I think we're going to start unpicking it a bit next week...
So, yes – I completely get this...it's an awful experience. But your therapist sounds lovely and she will have seen things like this before. My therapist was so incredibly compassionate about it – I think mainly because, at the time, she could see how much I was suffering. She said afterwards that it was 'brutal' and that, although on one level it may show some therapeutic progress as things are shifting and looking for a way out, on the other hand it was too much and that she wanted to try to avoid it happening to that extent again because it wasn't good for me to get re-traumatised.
Talk more with your therapist about what happened (not necessarily right now...you might want to wait for things to settle a bit more first) and look after yourself. Take things gently in the aftermath of this distressing experience.