It's okay! You can tell us your problems whenever :D
Confront her. Ask he why she wouldn't say that to your face and instead told you over the internet.
Ask her to confront her when it comes to things like that.
Tell her that that's what she has to put up with if she wants to be you friends. She'll have to put up with you bad days and your good ones.
Assure her that if it were her having a bad day, you'd deal with her, and you wouldn't mind, because that's what friends do.
Finally, if all else fails, slap her.
I hope this helps! :D