There is a component of dissociation in the PTSD. However, every human dissociates.

When you are driving, for a usual example, and the next thing you know you are at your destination but don't recall all the exits, streets etc, you dissociated during the trip. Some ppl call it daydreaming, when you allow your thoughts to take you away from the boring classwork... Or even when you are so focussed on a project that you don't hear (you tune out) the surrounding noises... that's also dissociation.

There is a sticky post at the top of another forum...maybe anxiety? Grounding Techniques. It gives things you can do to stay in the here and now, rather than dissociating. It can feel scary, but with a mild effect, usually quite safe.

I suspect something familiar in the dentist office may have triggered your mind into a connection with whatever caused your PTSD?

TC!