Why do girls give the cold shoulder to guys they like?

I like this girl and she was talking to her friend and she said she really likes me. This was after I gave her a note in class saying if you want to talk to me give me a signal. She did but I had to talk to someone else for a school project. She gave me signals before but I didn't know she might have thought I was blowing her off. Might have hurt her. But she's given me the cold shoulder ever since. But still looks at me and sometimes might even make eye contact for 3 seconds. But she just keeps trying to look the other way. What should I do? Also why is she doing this? How does she feel?
Why do girls give the cold shoulder to guys they like?
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