I've had flashbacks in session before, which sounds similar to what you describe. When I first started having them, I was at home. I told my T about it, he said it sounded like flashbacks. Often when we talk about difficult subjects I get triggered and I might have lots of memories coming up.

When in session, of course my T talks to me, he tries to help me manage my emotions and still talk about whatever is going through my head. We also do a lot of mindfulness.

When I'm back after a session and have trouble due to memories or something that therapy triggered, I can write it down for my T to read. Usually I'll also contact him earlier for a phone call or extra session, which tends to help me.