She turned me down, and now I don't know what to do?

I asked out this girl I work with (part-time job, we're both in college) and she turned me down, said she had a relationship"ish" thing going on. Anyway, part of the reason I asked her out is that I got the impression that she liked me. Now I know I may have been seeing something that wasn't there or I got the wrong impression; but up until then she always seemed so open and friendly, and we often teased each other, and even the way she looked at me and her body language suggested she was interested.

So the first few times I saw her/worked with her after that it was really awkward, she wouldn't really talk to me or acknowledge me much, she even seemed annoyed with me. But gradually she has been acting more normal and I think the brunt of the awkwardness is over. But it isn't like our friendship before, when we always joked around and teased and talked a lot. I NEVER meet girls I'm interested in and she's the first girl I've been interested in enough to ask out. How do I tell when a girl is just acting flirty for the ego boost or because she's genuinely interested? I feel so embarrassed and vulnerable around her now that I know she isn't interested in me the way I am interested in her.
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what's most frustrating about it is that I never meet girls I'm interested in like I was with her, not to mention I am shy and don't meet many girls to begin with. Girls don't go for shy guys, despite what they say, and I don't know how to not be shy.
She turned me down, and now I don't know what to do?
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