Need advice about this girl

I work at a restaurant and I'm a month away from being 18. Probably a month ago came this girl from college to work. I don't know her age but I think she is around 19-20. I dont think she has a boyfriend, but who knows? We got along well at first, just talked about school and friends and stuff. I really really like her, not just because of her looks (She is gourgeous) but also because of her personality. I havent met with her outside of the workplace yet.
Now a week ago she just started laughing at every single joke I said even though it wasnt funny, listened to everything I said and looked me in the eyes the whole time. Then she faked a note for another girl who works there, saying do you like me , yes no box. I told her I saw her faking the note and thats why i put yes (yes to her not the girl who she faked ). Then after that we joked around for an hour or so while working, and at the end she just laughed like crazy at my joke and lightly slapped me on my face (actually she just brushed her hand on my face).
All the obvious signs that she likes me right? Then this week she is completely ignoring me, she doesnt say bye when she leaves or say hi when she comes. She pretends im not there and only notices me when i say hi to her. She tries to never look at me and tries to avoid me. Mostly she just talks to other guys, and flicked me off once for saying a joke.
I am completely lost, since im going crazy for her and I have no clue what she is doing. Does she still like me? Does she not like me anymore? I really dont know what to do. Should I just approach her and lightly ask her out to go to a movie or something?

There could be a couple of reasons:

1. Maybe she has a bf now and she doesn't flirt when in a relationship?

2. Maybe she is done having fun with you... the novelty wore off?

She also might not date you because of the age thing. Most women around her age don't like younger guys.