My bird is acting weird.?

Ive had my cockatiel for about 6 months and he has never shown signs of aggression until now. he just recently started acting as if he is scared of me. when i open his cage he backs away and hisses and tries to bite me. Its annoying because when i come home in the afternoons he will chirp as if he wants attention, but when i give it to him he acts defensive. Why this sudden change of attitude toward me?