(Getty Images)

KATY PERRY seemed to be joking when she said she broke off that kiss with MILEY CYRUS because, quote, "God knows where that tongue has been." 


But it sounds like Miley didn't take it that way . . . and as a result, IT IS ON.  Because she fired back with a Tweet that insulted both Katy and her ex-boyfriend JOHN MAYER.


She said, quote, "Girl if you're worried about where tongues have been good thing ur ex boo is ur EX BOO cause we ALL know where THAT [tongue's] been."


She might have tried to smooth things over, though, because her next Tweet was a picture someone had DRAWN of the two of them kissing, and Miley captioned it, quote, "Don't you act like you didn't love it."