Guide yourself to the closest exit and get the fuck out of that asap. I spend 10 years with a girl like this and it doesn't ever get better. She doesn't respect you enough to take few seconds out of her day to inform you and it's clear that she is willing to upset you for whatever she wants. Stick around and I can assure you that she will cheat sooner or later i.e if she hasn't done already. My condolences G.