Why doesn't my girlfriend ever invite me out with her friends?

We've been dating a year, and she now rarely invites me to join when she goes out to the bar or has people over. When we do go out, she seems indifferent to my presence, despite my efforts to have a good time and involve her. Sometimes she even gets mad at me at the end of the night for not paying attention to her! When I ask to join, she says that she doesn't care either way. I do have my own social life, and I almost always invite her. I've tried talking to her about it, but she gets aggravated, and doesn't give any answers. What should I do?
Why doesn't my girlfriend ever invite me out with her friends?
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