I ask this because there's this shy girl and we talked a bit then a few days ago, we talked a lot and she opened up a bit. I didn't see her again until 5 days later. I was in the hall, and she came out of a classroom, looked at me, quickly looked away, and then as she walked by, looked at me, smiled and said hello.
Some shy people find eye-contact intimidating, especially when they care about what that person thinks of them. The more she gets to know you, the more she will open up. The fact that she smiled and said hello was a good sign.