Girls, Why she keep acting this way?

Me and a girl keeot starting at each for months. I then decide to talk to her for the first time and she started smiling to herself?
Then she keot making excuses to talk to me and would try to be nice by letting me use equipment she was using. Also when ever I was in the gym she as there.
But I noticed she keeps look bothered if I'm around other girls. she tries to talk to then and pretend they are friends?
Or if she catches me lookkfn at another girl she stares, then tries to make me jealous by obnoxiously staring at guys
Girls, Why she keep acting this way?
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