Is it normal for a girl to reject a guy she likes?

So, I'm shy. But a some months ago however, I had a moment of madness that would come to put a hold over my romantic interests until just a few days ago.

I was walking past a coffee shop, feeling pretty confident and good about myself after a nice swim. As I walked past the place, I happened to look over at the girl that works there (she'd casually smiled at me in the past). On this particular day though, we locked eyes for a moment, both smiled and then carried on what we were doing.
The following day I went to get a cookie ('cos I'm too cool for coffee). She served me and tried flirting like crazy, me being shy though (and not feeling so confident) completely closed up, and got out of there as quick as possible.
After then, it became a sort of daily routine for me to get my cookie. She never flirted in quite the same way, but she always seemed VERY happy to see me, so in general things were going we'll enough to keep trying (always thinking "maybe today, I'll speak to her properly").

So four months of mild flirting and cookies later and were familiar with each other, but I'm still too shy to flirt, or hold a solid conversation, so I decide I need some pressure. Taking advantage of my half-crazed, exam-stressed self I decide to ask her out. Goes like this,
*says hello etc*
"Are you busy tonight?",
"Yes I am... Thank you though".
So I smiled sweetly and walked away, quite impressed with how calm I remained through the whole thing.
Over the weekend though, I thought about it and decided was going to show her how not bothered I was (kind of hoping she'd cold shoulder me).

So, today I went to get a cookie again, this time timing it so to have her colleague serve me as oppose to her (I was still feeling a little sore). She comes out as I'm in line and quickly takes her colleague's place so she was now serving me. As usual, she was all (big) smiles etc. So now I'm totally confused. Is it normal form someone to seem so interested but not be?

(I don't think she's just a flirt either. I know a girl like that, and she's really awesome, I fancied her too, but that's something else entirely. Anyhow, both girls are completely different so I think there more going on here).