Why does this shy girl do this (about eye contact)?

When I actually get to talk to her, for the most part, she'll look straight into my eyes and hold the stare (this eye contact doesn't always happen when we are talking to each other, it just happens more frequently). A couple of times, her eyebrows were raised and she got wide eyed. The majority of the time when we talk, she'll give me one word answers and then quickly look away. Sometimes she doesn't even seem interested. On other occasions, she'll be looking at me when I'm not looking at her and then when I turn to look back at her she hastily turns her head or darts her eyes away from me. She makes it look like she wasn't even looking at me, even though it was obvious.