It's like Dr. Jeckle and Mr. Hyde

I don't know about you, but I have a friend who over night has practically turned into someone else!
I mean, there's a difference with someone changing, but usually it's after a long period of time, but this is literally a day or a week even.

It's really weird because all of a sudden they're being really nasty, and rude, but they weren't like that before. She explained that she was "just joking", but she never said things like that before, and pretended it was a "joke". It's like, I get that you're joking around, but you're kind of being an ass and you weren't like this to me before....so what's the deal?!

What we had going before was good, but how you're behaving to me now is going to make me stray from you. Eventually we'll be someone we used to know to each other, just like that song from that guy I don't know the name of!
I really don't know what to do, because they're kind of getting on my nerves now, and I might say something mean back. Which definitely will not be a joke, and that might ruin our friendship for sure.

In the comments below, say what you think should happen? Should I say something to my friend? Should I just be quiet and see what happens? Should I just shut up, and pet a potato?
Also, tell me your experiences.
You can also make a post on my forum. SKC-When friends turn into something else? <-Click here

Here are links to the entire Mystique Series!
Part 1 - Part 2 - Part 3 - Part 3.0 - Part 4 - Part 5 - Part 6