Why is my cockatiel acting like this?

I've had him since he was 6 months and he is now about a year old. So that's six months we've bonded. He would chirp a bit and i could tell he wanted to get out his cage so he would come to me when i open his cage and hop right on my fingers. Now when he chirps and I open his cage sometimes, not all the time, he hisses and acts like hes going to bite me. If he does that i leave him alone, but sometimes he acts normal and jumps on my finger. ill let him on my bed or the floor to discover and get some exercise but if i put my hand anywhere near him he hisses and bites and backs away. I just dont see why hes had this sudden change in attitude towards me. can anyone explain? THANKS!