Why does she avoid looking at me?

I'm in college, a senior. There's a girl in my class who never looks me in the face. Its like she avoids looking my way. We were in a group discussion and she looked at everyone else but me. I'm friends with one of her friends and when we talk, she's always silent and never has anything to say to me. I've never done anything to her. I actually wanted to be her friend at one point in time. It hasn't always been like this though. She used to be cordial withe but not she just completely avoids me.
Why does she avoid looking at me?
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