What to do about a text?

on sunday i found a text message in my fiances phone to one of her girlfriends that has a rep as a slut stating i need to go out and flirt with guys so sick of being a good girl and want to be bad. I was caught off guard because she doesnt seem the type. she started a fight 2 days prior about my friends new girlfriend who i haven't even met yet. she thinks i'm going to be hanging out with her younger crowd. i've been loyal our entire relationship. I'm a homebody and only time i go out without her is to go play softball. alcohol and her that fight she picked are her justification for sending that. I'm being treated like an *** & apologies seem insincere with no regard to my feelings. am I overreacting? furhther complicating things is today is her birthday. she wont answer my calls or texts because the last couple days i've been to upset to talk.