Why is my budgie acting this way?

I got my budgie a week ago. He was at first VERY untame and HATED the sight of people and fingers. Now, he doesn't mind people and eats on my hand. He used to be so scared that he didn't even let out a peep.

Recently, he's been screaming at me randomly...

such as...

I'd be watching TV with him, and he would start pacing up and down my shoulder, on to the desk, on the keyboard, to my lunch box, nibble on it a little, climb on it, then back on to my finger, then to my shoulder, back to the desk...

then he would get chirpy... like very soft at first, circling around a little, then looking directing at me and his chirps would get a lot bigger until he starts doing that angry budgie sound that goes "AACK! AACK! AACK! AACK!!" and starts flapping his wings in place.

Then I give him some attention and go "What's wrong?" and he starts chirping again and then gets quiet again.

WHY is he doing this?? He does this so randomly and out of nowhere sometimes, it alarms me.