Hi hummingbird. Love your name!
I'd check out with your regular GP just to make sure it's not physical. Somethings meds combined with well almost anything...heat, stress, diet etc. can cause a weird reaction. Even, in fact, sleepwalking! I learned not to self dx. So you really don't absolutely 100% know you were dissociating. So you don't have to describe it as such to your new therapist. Could be a one off even if it were. May never happen again. Know it's upsetting & scary but unless it becomes a problem or frequent or is a medical issue not something that needs reporting on the first visit. Normally I'm a tell T everything person. Experience taught me that if T doesn't know what's wrong, can't help fix it. Like taking car into mechanic & only telling him 'it's broken'! It can be expensive too, I learned! But I seem to prefer to do things the hard way!! Good luck with your first appointment. Hope it goes well.