my bird flipped, why??

I've had my green-cheek for 4/5 months now, hes great and a real sweetie (most of the time), but lately, I can't do anything with him. Can't pick him up, can't feed him without him lunging after my hands, can't play with him, can't even get close without him puffing up in aggression...

Sometimes he'll let me put my hand on his back, but the second he sees a finger its back to attack mode. I really have no clue what triggered him to be like this, I play by all of the rules (no yelling, no roughing, no pulling away, etc..), but now its been a week since I've been able to handle my bird and I'm starting to get very sick of this behavior.

Any Advice?

Please don't be too frustrated and don't give up on him. It truly is a phase he is going through. I remember this phase with my green cheek. He was so sweet and then he "turned".....I just absolutely wondered what I would do with this bird. I didn't want him to be caged all time but he was vicious.
Just open your cage door and ignore him. It sounds as if he was pretty bonded to you until this happened. He may have pin feathers growing in and they are sticking him. That is what I finally decided. You really shouldn't be touching them on the back anyway. Forrest will let me cup my palm around his back but he hates fingers on his head or back. Just doesn't care for petting. If you just let him do his thing, he will eventually want to be with you and do what you are doing. JUst keep him in the area where you are. Trust me, green cheeks can't go very long without you. Forrest does everything that we do. Oh and one more thing, don't reach in his cage to get him....always let him come out. Watch his body language and learn what he is saying and you can avoid a lot of bites.
Don't try to force the issue, just wait until he comes to you. Don't use fingers, for some reason my green cheek doesn't like fingers either. I am so glad I didn't give up on mine, he is the love of my life and you WILL get through this. Time and patience and lots of LOVE. Don't punish or yell, talk very sweet to him at all times, it has a calming effect on them. Whisper or in a low sweet voice tell him to be a good boy. It seems they go thru phases sometimes, for days at a time, then I go into ignore mode....he can't stand it. They can just be little "Butts" sometime. When he comes to you, tell him what a good boy he is. Praise and also give him treats from your hand. Hope this helps and keep us posted.

Yes yes! Molting! Chichi was like this. He was great then tore my hand to pieces. I'm still healing. but he's better and I found tons of huge painful pin feathers underneath that I couldn't see. Using dowels helps a lot. When I would try to get him to step up he would bite, but if I used a dowel first he'd realize he couldn't bite it to bits, then I'd move him from the dowel to my finger. I would t try to pet him, just talk sweetly. Then while things were good, I put him back in the cage. If we had a bite, I'd wat til he was being nice again before putting him away so he didn't think it was a way to be left alone. Good luck!