My girlfriend acts different around other guys?

my girlfriend is normal quiet and reserved around other guys when I'm around. but when I leave she puts on this happy laughy and peppy personality. she tells me she's not flirting but that's how it make me feel. it wouldn't be so bad if that's how she acted around me but its just when I leave. I've tried to talk to her about how I feel but she just tells me that she's not flirting. what should I do?
My girlfriend acts different around other guys?
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