Girls, Why she hide her boyfriend from me?

After 4 weeks of flirting such as: staring, listening in Which resulted in her getting jealous because I didn't approach and instead spoke to other girls. she made extra effort to look nice, and was waiting to print in the library. I was studying. She was really nervous and didn't know how to stand and Kept fidgeting then tried to sneak a look at me. And looked away fast like she didn't know to look or not.
5 minutes later she saw me talking to another girl and seemed shocked. When I returned to my desk in saw her out the window whispering to herself, red in the face looking upset.
The week after she purposely stopped looking back. 2 weeks later I approached her to see what's up

1st time - we was nervous , she was nice and wished me happy birthday. But she couldn't make eye contact and was quiet. Her friends was nice and she laugh at my jokes. But she was looking at me out the corner of her eyes up and down So I left. Then she turns from being introverted and quiet to being happy and loud with her friends.
Later She was talking about me to her friends
2nd approach - she made more eye contact , was more talkative towards me and kept twirling her hair constantly. Then later on she was talking to her friends about me again and kept sneaking glances when in class
3rd time. I sat down and faced her an her friends but she was still shy. I wanted to make her comfortable so I complimented her a few times but was really nervous and her friends noticed. but pretended to not notice 10 minute later I was late for class and said bye to her friends and good luck for test.
she straight away talking about me to her friends then i look at her waiting for elevator and she stops talking and we both start staring at each other and she kept looking at me everytime she saw me. Why is she so shy and keep staring , talking about me and get jealous when I talk to another girl and sad if I not approach her..
+1 y
The week after i was too busy so didn't approach and her friends kept staring at me like never before.
I seen her make more effort in appearance and she was studying. She kept sneaking looks when I was returning books. Then i faced her direction to talk to my friend and she looked me in the eyes like she's upset that I didn't speak.

But I found out she has a boyfriend which she doesny know i know and she hides it from me I do like her but I dont understand why she does that
Girls, Why she hide her boyfriend from me?
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