What is this girls deal? please help?

me and this girl have been friends since the 3rd grade and now were about to be seniors in high school. even though were friends she wont add me on facebook but whenever i send her a email she replieds back within a minute or so. i have noticed that whenever i see her at school she is usually lookin at me out the corner of her eyes. i have also noticed that she will walk past me in the halls for a number of days before stopping for awhile. she also seems to have trouble tellin me hi in public and so far she has done it once but it took her four days to do it. whenever i asked her about any of this she claims to have no idea what im talkin about. i have asked her out twice and each she told me that she had plans which was no lie. please help what is her deal