Girl's Behavior

When a girl is nervous around a guy all the time?

I have this classmate. She is a nice girl, a bit shy but she can be pretty chatty and pleasant with people. I have known her for months now and doesn't matter how many times I talk to her, she just looks so nervous (fiddling with things, looking at her phone the whole time etc). She isn't like this with anybody else and she will pretty much hold conversations about any topic with other people. When I try to talk or make a joke she just gives me all this sarcasm or smartass answers or criticizes my taste in whatever I'm talking about and it just makes me feel uncomfortable. The only way I can get her to loosen up is by being a bit childish and annoying, I can get her to giggle and to give me these really wide smiles, pretend she is busy with her phone and then take a quick look at me while she smiles and then go back to her phone. She is 20 and I'm 28. there's an age difference but she gets along with guys older than me and she doesn't act like a nervous wreck around anybody else. What can I conclude from this? Any help?
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Ive asked her why she acts different and she would deny it at first but now she just tells me "because you are annoying" or asks me "well, why do you care so much?" How do I respond to that? Lol
When a girl is nervous around a guy all the time?
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