Shy girls, why would you be mean to a guy?

There's this girl who is the shy/quiet/sweet type. I like her quite a bit and at first she was nice to me but then little by little she started to turn into kind of a sarcasm machine, she likes to tease me (she has actually admitted that to people when they ask her why is she like that with me) and we always engage into these playful arguments. She is not like this with anybody else, she talks and jokes with them but not to this extent. She can say some pretty mean things to me (they are playful and funny so I don't mind it) and although we argue and tease a lot, we know a lot about each other and she seems to remember little things that I've told her weeks before (she likes to bring up things I did or said and make fun of them). So my question is, what exactly makes a shy and sweet girl act this way towards a specific person? SOmetimes I get the feeling she doesn't like me and I've asked her and she says "if I did, I would've told you". She just seems so nervous when I'm around her and so relaxed around others it makes me think otherwise. she can say things like "I'm laughing at you, as usual" or "you are so annoying", so, any ideas?
Shy girls, why would you be mean to a guy?
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