OK, I fully expect the more experienced to give me a well-deserved "Lighten up, Francis" here, but here goes:

I'm brand-new to the hobby, just had my first experience two days ago.  The provider could not have been better - professional, pleasant, etc.  I plan to write an extremely favorable review.  But during and since then, I have had overwhelming feelings of dissociation - feeling completely detached from the experience.  It's been 48 hours, but I barely remember a thing from that day.

Perhaps a defense mechanism to deal with the stress of the event (I'm a bit high-strung, and was very nervous)?

Has this happened to anyone else?  Did it get better with experience?

With all respect -- and I mean that -- if it persists or repeats you should find a good therapist. Sometimes this kind of thing can result from triggering long forgotten traumatic memories. While that might very well not be the case for you, the best thing you can do is address it sooner rather than later.