Why is this girl rude to me?

My boyfriend introduced me to his group of friends... apparently before my boyfriend knew me... he approached one girl in that group... he tried to get intimate with her while drunk I understood... but she rejected him..
So when. Join the group I did not know all this.. the first day I met that girl she told me smth rude about my boyfriend with an authoritative and was shouting for every little things. Why would she do that?
Why is this girl rude to me?
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