I talked to this talkative/sociable girl for the 1st time today, and she could not keep eye contact at all & seemed nervous about talking to me. Why?

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She could have been attracted to you, you could have also been really awkward. The two are not mutually exclusive and neither precludes other possible explanations.

This sort of thing is not anything to obsess over, just keep talking to her and the awkwardness will go away soon enough, to be replaced by whatever other silly thing you do to turn her off.

Expect to go through this a lot until you build up the confidence and social fluidity to be able to navigate these situations well.