I have a Green Cheek Conure who sometimes gets grumpy like this. She will go through long periods of cuddling with me and happily flying to my finger, only to change her attitude for a few days and try to nip at me anytime I try to handle her. She always comes around, though, with patience. Your birdie may just be feeling a bit moody. :)

He might be hormonal, wanting to pro-create.

I have a year and a half year old sun conure, I've been in his life most of his life (he was at the pet store I work at) and he's lived with me for six months. He's mostly been only interested in being handled by me, and can still be a little nippy every now and then, but I'm use to it and love him anyways. The past week he's been more antisocial than usual. He doesn't want to come out of his cage when I go to get him, he screeches at me when I bring my hand towards him. He seems more quiet than usual, he still calls out and talks to me, but even when he's out with me he's less vocal. I know this is a dumb question, but I was wondering if anyone had any ideas of why he's acting differently? I took him to the vet 6 months ago and he was healthy, and I will probably take him to the vet again soon but I was wondering if I'm overreacting. I've wondered if he's angry at me for something or unhappy about something if he's not sick. He shows no other symptoms, no fluffing of feathers, no tail bobbing, no feather plucking, still eating and grooming, but yes I know how birds hide their illnesses. Any ideas?