My girlfriend gets moody and annoying as hell when she's on her period...what should I do?

I love her and she's extremely caring, sweet, funny, nice to talk to, intelligent...but gets annoying, moody, rude and ironic and etc when she's on her period...I'm not even exaggerating...it bothers me to the point that it makes me not want to talk to her because I'm always attacked...help.
My girlfriend gets moody and annoying as hell when she's on her period...what should I do?
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