She stares at me somedays, then acts like I dont exist other days?

There is a really popular girl that has always been quiet around me. some days, she stares at me, but looks away quickly when i look back at her. she follows me just to pass me in the halls some days too. Her friends stare at me too. Its just that other days, like today, she acts like i don't exist at all. I see she has no problem talking to other guys, and doesn't seem so shy around her friends too. It is frustrating because i think she likes me, but then i dont think she actually does because she doesn't seem to notice me at all. But then randomly she will stare at me again and so on. What do i do?
She stares at me somedays, then acts like I dont exist other days?
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