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She can absolutely be nervous because she likes you and sexual tension is a great thing. So if you take a look at other IOI's you can get a better picture. I just wanted to point out that her playing with her hair shouldn't be taken as an IOI all on it's own. If she's creeped out and uncomfortable, she might still do it

It think it depends on how shes touching her hair, too though. If she does have her arms crossed, and shes twirling her hair in her fingers and showing negative body language, then the hair touch could simply be a self comfort thing.
However, if she runs her fingers through her hair, or takes out her hairband to re-do her hair, shes grooming herself and trying to make herself look better, which is an IOI, simply tucking her hair behind her ear would have the same affect.
As you say though, and as many PUAs advertise, you should see at least three IOIs before moving in for a kiss close or whatever. When I'm out with my girlfriend, I've noticed that when shes talking to anyone else, she will not touch her hair, at all, but the second I walk over to her, and start talking to her, she cannot stop touching her hair.
I've found that quite interesting and weird at the same time, considering its a subconcious thing, or perhaps because me and her are both interested in body language, and we read people together, shes making a conscious effort not to touch her hair when talking to other people, who knows haha