The singer appeared as Renee Rapp's special guest Sunday during the music festival -- and sure enough, she sang her biggest hit 'Tik Tok' ... which starts out by name-dropping Diddy, but which she switched up in emphatic terms as she took the stage.

Instead of singing, "Wake up in the morning, feeling like P. Diddy" ... Kesha sang, "Wake up in the morning like f*** P. Diddy" -- and Renee was right there with her to sing it loud too.

They didn't just say the words, but threw up some middle fingers to drive the point home.

Kesha continued on with the rest of the song like normal, but this appears to have been her first chance to sing her signature track on as big a platform as this -- and she certainly made clear how she feels about Diddy and the controversial saga surrounding him right now.

Of course, this isn't the first time she's altered her intro lyric as it pertains to PD. Remember,  she did the same late last year when Cassie sued him ... nixing him entirely in concert.

The reason that's important is 'cause Kesha is a well-known/outspoken advocate for victims of sexual violence -- which she herself has claimed to be in the past ... infamously so. As we all know, Cassie alleged Diddy had subjected her to a lot, even though he denied it.

Now, in light of the fact Diddy is at the center of a federal investigation ... Kesha's made the leap from not saying his name to saying it with a caveat attached ... screw this guy.