Rihanna Gets Into A TWITTER-WAR . . . With Chris Brown's GIRLFRIEND Karrueche!!!

We're not quite sure WHY Rihanna's cousin still finds a need to try and clown Koochi Koochi . . . but she's doing it . . .

And after she did it, Rihanna co-signed the messy post on Twitter. Karrueche being the LAME that she is - said something about CLASS. Ain't nobody got time for that . . .