Why do some girls do this?

There is a girl who I like very very much and secretly admire, as for me she is just gorgeous, this is one of the first times in my life this has happened where when I see her my heart flickers and love almost everything about her. Her hair, Her skin tone, her features and she knows I like her... but I'm not sure if she knows how much I like her and I want to tell her how I really feel as this is regulary causing me to be tired and sick emotionally. This may sound cliche from the movies but I am more of an old school gentleman (I am 30) and want to tell her she is beautiful and how gorgeous she is.

I know that I can't as we have not known each other long. I have known her since last year seeing her 1x week in College. Then we were on a Class trip for several days and we got involved with each other, going places just us two together like visiting shops, swimming together in the moment.

I asked her out and she said yes but didn't meet up as nothing happened as I and others believe it was too fast and unconfident or didn't feel right to her at the time.

she likes me in person and bad at texting (said she is a bad texter)
She didn't reply in 3.5 months and when we saw each recently, she would walk in quietly and secretly have a quick look at me and not speak to anyone and even me. She opens up very slowly and talks to some people.60% she is extremely shy quiet type and 40% loud fast paced and extroverted.

Sometimes she does walk up to me and talk and other times say nothing and I would feel a presence in the air or electric in the air and soon as I say hello she instantly replies and faces right at me, looking Shy.

Recently she told me she is feeling tired and needs to go home and hates she works too much. That day she wore a lovely half dress under a coat and earings which she never wears to class. Before she left she brushed her hair behind her ear showing clearly her earings. She asked my timetable and said okay these days are the days that we are together.
Why do some girls do this?
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