umm... Do I know you?

Okay... I don't know many people up here. And those I do know I will know to recognize for sure, cause I know them well.
However, there is this girl on the bus from my place that seems to know me. She waves to me when she gets on the bus, and then whenever she sees me around the university she says hi. I, of course, return the salutation, cause I'm nice. But I have no clue who she is. Mind you, she looks really familiar... but I don't know who she is.. and its driving me crazy!
I'm much too shy to ask if I know her, and that's just rude. Maybe she's just being friendly... I really don't know but its getting quite frustrating.
But oh well... I'm sure I'll figure out how I know her eventually.
Back to work.