Rousey and the Bella Twins were part of a 6-way tag team match against the Riott Squad on "WWE Raw" in Chicago ... and things were going great, until Rousey got double-crossed.

After the ref had raised their hands in victory, Nikki hit Ronda with a clothesline from behind ... knocking her to the ground.

Brie Bella then rushed over and continued the beatdown -- stomping her out and throwing her into the steel ring stairs.

Yeah, we know ... fake (blah, blah, blah) -- but watch the video, Ronda's body took some real damage here.

So, what happens next?

Revenge. Sweet, sweet revenge.