Pop sensation Rihanna, who reportedly drank beer on the day of her grandmother funeral, has apologized to her granny for the immature act. The star posted a photo of herself sans make-up and holding a bottle of beer on Twitter and wrote ''I had to!!! Sorry granny.'' The picture showed an exhausted Rihanna, who didnt even try to conceal her tired eyes. Rihanna attended the funeral, which took place in Barbados, Caribbean, wearing an all-black outfit. She was spotted coming out of a chauffeur-driven limousine with her head bowed to the ground. She also cancelled a performance at Spain's Rock In Rio festival.