I have a 8 months old lovebird. After a few weeks after I got him, he was already used with my presence and he was letting me pet in him in his cage, he was letting me take him outside and he stayed on my hand while I was talking to him. About one month ago, some things that may have changed him happened.
First: I changed his cage, tough I don't think this could have made him aggressive. He seems pretty happy is his new cage.
Second: In one of the mornings, my grandma got mad because he was chirping too loud and she may have slapped him or something because that day, he kept on startling towards my hand. At the end of the day he was fine and he let me pet him but a few days after, his behavior changed again.
Since then, every time I put my hand in his cage with the intent of petting him, he bites the crap out of my hands. He squeaks and startle towards my hand even since I get my hand in the door of his cage.

I don't think he is unhappy because he is "held like a hostage" because 70% of the day, his cage door is open and I can say he gets out of his cage quite often to fly around.
I am feeding him regularly, changing his cage lining every day and washing his cage every week. I don't think he has a reason to be upset. How can I regain my trust and friendship with my lovebird?