<human>: I've had this cat for 4 years and she's always been a sweetheart, but there are just certain situations where for no apparent reason she will start FREAKING OUT and attack the shit out of my hand.

I've had cats all my life, I know the warning signs, I know when a cat is getting obviously annoyed, this is why I'm confused.

She does not hiss, she does not whip her tail around, she doesn't do anything except lay there purring and happy. But sometimes out of nowhere she'll just take a big bite out of my hand and start scratching the shit out of me.

What the hell is causing it?  Every time it happens I immediately grab her by the scruff of the neck and just stare at her. She gets (what appears to be) deeply ashamed of her behavior and won't look at me. She seems to understand it's bad behavior, so why is she doing it?

Or why won't she atleast warn me she's getting annoyed? She goes from happy purring to "HOLY FUCK CRISIS MODE ATTACK ATTACK ATTACK" in a matter of half a second. 

I don't understand what's causing this.

Can you succinctly summarize the text in 1-3 sentences?
<bot>: My cat is normally a laid back sweetheart but occasionally goes off like a crazy bitch and attacks my hand with no warnings