Knowing People

Ok, I'm working on some homework for PR class right now and this girl
comes up to sit next to me and say hi, the catch is, I have no idea
who she is, even though she seems to REALLY know me. I ran into her
once before at the bookstore while buying books and she was waving at
me furiously then like she'd known me for a long time. I feel
terrible not knowing who she is and I really don't want to let her
know that by asking, "uh, how the heck do i know you?" Apparently
from our brief conversation just now, she's in a couple of my
classes, so i'll have to pay attention I guess.