What does it mean when my girlfriend doesn't invite me to parties?

My girlfriend and I have been dating for about 3 months. Things are going very well relationship wise, but this is the only thing that bothers me. I don't embarrass her, I'm usually pretty quiet during parties. I also feel like I have a pretty good relationship with her friends so I'm confused. Then a few days later after the party she will tell me all about it and how awesome it was, making me feel like sh*t.
What does it mean when my girlfriend doesn't invite me to parties?
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