My girlfriend has been acting very nonchalant and less caring to me. I Dont know why she's acting this way?

I've spoken to her to figure what's wrong and she responds with "I'm fine" or "nothing?" Like she acts bipolar against me I don't know what to say with her she even had the audacity to think about other men etc.
My girlfriend has been acting very nonchalant and less caring to me. I Dont know why she's acting this way?
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