The following video of a twerkin' Rihanna will probably upset Chris Brown for two reasons:
1) It's sexy ... and she's reportedly cut him off from tappin' that ass.
2) She's twerkin to a Drake song ... Chris Brown's mortal enemy.
The footage was posted by one of Ri's friends -- showing the pop star gettin' down to A$AP Rocky's song "F**kin' Problems" -- which features Drake.
It seems to be a thinly veiled dig at Chris -- considering he and Drake hate each other so much, they brawled at W.i.P. nightclub in NYC last year.