Sometimes she seems nervous/shy, sometimes not. WHY?

So there is this girl and sometimes she's really weird around me, like nervous, fidgety, and shy. But other times she's more playful, bold, and flirty. Both "moods" seem to indicate that she may like me, but why does she have both? I thought it was one or the other. Is this common, and can this mean that she likes me? I mean, are girls sometimes nervous around the guys they like and other times not?