I had a very similar experience to yours, Hello, about a year back...but I think mine was worse.
I thought the girl I was making out with (who I knew well, but wouldn't have done that with) was someone else, but I only realized that when I asked her a question about the other girl's best friend & she didn't know who I was talking about. It completely spooked me too, and I had the same feeling where for like a minute I couldn't figure out who she was (I forgot more than just her name though), but since she knew me full well & I didn't want to freak her out I had to figure out & deal with everything on my own time. I'd had some vodka just before, but I doubt that was a major factor.
I'd agree with Ceili that it's probably just a brainfart, and possibly was helped out a bit by the alcohol. I doubt it's a sign of any kinda long-term memory issues, though.
'Twas weird.