After reading some comments about Annie’s change in attitude over Kat finding somebody, I realised there’s another reason she could be upset over what she saw.
With how excited and encouraging she was about the love letter, I think Annie really wanted to be there for Kat through it all. Supporting her with first dates, what to wear, all that stuff.
Instead she hears nothing about the letter afterwards and finds out weeks later that her friend had been doing all of that in secret, without her. She must feel pretty terrible being left out of her friend’s life like that.
That said after all this build up, it would be a bit of a cop out if Kat having a girlfriend was a complete non-issue for Annie.