Girls: what does this mean...?

there's this girl at college im good friends with . we always do homework together, travel together etc.. and yesterday, we were on the train and she noticed something in my hair...like dust or something so she flicked it off with her finger and kinda smiled at me..then looked away shyly. wow. those few seconds gave me a shiver down the spine.. it was really weird. then we talked normally. why do u think she smiled ?. it was kind of a shy smile..