What do this signs mean? does she like me but is afraid to admit?

We have known each other since the 3rd grade and now were both about to be seniors in high school. i have noticed her doing a number of things that i can't really understand. for starters even though were friends she will not add me on facebook but has no problem with me emailing her. she also seems to have a problem with saying hello to me when other people are around. whenever i ask her about this she claims to have no idea what i'm talkin about. i have also caught her lookin at me a number of times durin school. she has also gotten nervous a few times when she see me walkin towards her in the hall. the last thing i don't get is that she will walk past me a number in the halls but if i was block her on facebook or something like that she will just seemed to vanish. what is her deal please help