Beaten-up beauty Rihanna donned a bikini and soaked up some Mexican sun a day after her 21st birthday. The shocking bruises that sources say boyfriend Chris Brown left on the songbird’s face just before the Grammy Awards were not visible in exclusive photographs obtained by the Daily News (see print edition for photos). Nor was she smiling. Her grim expression, so at odds with the balmy setting, hinted at the heartbreak the “Umbrella” singer has endured since her private agony became public knowledge. Rihanna’s family was furious after a photograph of her battered face was leaked, and the singer was determined to get out of Los Angeles. Shortly before she split last Thursday, Rihanna released a note to her fans telling them “she remains strong.” Then, with a couple of close friends, Rihanna decamped for a private beachside villa in Punta Mita. Brown, a 19-year-old R&B singer, was arrested on suspicion of making criminal threats against Rihanna after the Feb. 8 attack. He has since enrolled in anger management classes.