It was someone you know, so that takes the stranger anxiety/shyness thing out of the equation. Since you know this person, I would interact a few more times and gauge the response. If it is still aversive, and that is different from past behavior, then I would start to wonder. If it is a one-time thing, it could be something as simple as that person just not having a good day and not wanting to encourage interaction by shooting you a smile back. I would say don't let it bother you, but if you posted a thread on the topic, I'd say it IS bothering you. Good luck on figuring out what that is about.