Yep, I get this too from derealization. I start to feel uncertain if I imagined the person/interaction, like maybe they were a character in a book, or I dreamed of the interaction. Facebook and electronic communication has helped me with this a lot. I can see text transcripts, profile pages, etc. "proving" they're real, etc. I've never been given an explanation of why this happens, but the answers provided above sound right.
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