Am I her backup plan?

I don't know, there's just this girl I've become really good friends with, and while I know that she has a boyfriend I can't help but feel that she flirts with me a lot, lets me dance close when out drinking, wants me as her 'fitness-date' etc. I haven't had a girlfriend or even an actual close female friend before so I don't really know if that's just how some girls are; 'flirty by nature'? I've just gotten some idea into my head about me being like a backup-boyfriend for when she lets the other guy go (she's mentioned concern for him because of his friends and also told me that she didn't have any potential picture of them marrying [yeah, maybe a little extreme to consider]).

Am I over-analyzing or is this possible - I really like her so I don't know how to act accordingly, wouldn't want to make things awkward if this is just her way of being friendly :S Please help me!
Am I her backup plan?
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