Why is my friend randomly being so rude to me and picking a fight?

So a few days ago I was having a normal conversation with my friend, we were texting about school stuff but then randomly she got mad at something SHE brought up and is now basically taking it out on me? Like first she unfollowed me... for what? I have no clue. Then the next day I was thinking like "it was probably just something in the moment" because ngl she always kinda does this but usually in a joking manner, so I texted her about something and she just replied very brief response, so I responded like "ok, nvm then". AND THEN omg she says "don't talk to me", like? I left that on read because what else was I gonna do, didn't even plan on texting her again but then she texts me to complain about her parter for this assignment, who by the way is also one of my friends. At that point I was kinda mad and just said like "oh now you wanna talk to me", and then she responds that I'm always taking the other friends side... Like I don't even know how all this came up but I just gave her a piece of my mind, basically saying that I'm not the one who started being rude and I'm not gonna talk shit about anyone bc its your guys problem. And now, omfgggg she's like, when did I talk shit you're just overly protective. I actually got pissed off by that, haven't responded yet because what do I say to that? Sure, maybe its not technically "talking shit" but she gossips about people A LOT, and not in exactly a good way (including said friend ahem).

Anyways, what do I even say at this point, like I need to show her that I'm not just gonna be there when she wants to complain or that I'll just brush off her bitchy behavior when shit like this happens.

Why is my friend randomly being so rude to me and picking a fight?
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