Have you ever gone through a depersonalization or derealization experience?

About a month ago I was studying late at the library, and had to go to the washroom. It was on an empty floor and I was feeling slightly uneasy. While I was in the stall I started to sort of “zone-out” and daydream for a minute or so. When I came out to wash my hands I looked up at my reflection and what I saw was quite frightening. I felt as if I was standing across from another person, and “my” (their) expression looked like an eerie, dangerous smile. Although I was not even smiling myself. It grew more frightening as I stared and I almost felt like the person staring back at me had violent intentions. Then I snapped out of it and quickly went to a more populated floor.
I had no idea what had happened to me until about 2 weeks later when we talked about the dissociative disorders in my abnormal psychology class (psych is my major). Apparently it can happen when under extreme stress. This pretty much explained it, as the episode happened during some pretty intense exam stress. I’m curious to hear everyone else’s experiences.