I do this all the time. When I think a guy is cute, and we make eye contact, I look away really quickly, and blush. And yes, its because I'm nervous, and a very shy person. This happened to me yesterday, actually. =)
When I like a guy, this is usually a dead giveaway. Just curious, are you still in high school, because if not then this may not be the case.
I would say that she's probably just shy. I know cause I can be shy and I do this sometimes. A sure way to tell is if you find her looking at you and she quickly turns away but then you find her looking at you again. You should be able to tell by the look on her face or if she is nervous that she begins to fidget or something.
It also just depends on the woman though, because the bolder women aren't afraid to look into a guys eyes for a long time. And the shyer ones tend to look away.
hm I don't know I'm shy so like the guy doesn't even have to be cute for me to look away lol D: I like eye contact that are intense but at the same time its just my body reacts and turns away ._. but iits true if he's uglier I do look a little longer than normal and I give him the "Wtf-Are-You-Looking-At?" face then I turn away x)