New Ball Python Owner

[NNL]

Hey guys,

Sorry if I'm posting in the wrong section, I'm quite new to this whole forum thing.
I've been reading up on this website for awhile now, and thought I'd join!

I've always been fascinated with snakes but too afraid to own one.
I gave myself a year to think things through to see if I was committed and could provide the care she need for her lifetime.
I decided to go with it, I had been sure of it. Anyways, I recently got a ball python from a local reptile shop last week.
She's just a regular and I was told she's under a year old.

I have a tank set up for her with everything (temperature, humidity, water, hide box, etc.) as recommended.
I'm fine with holding her, it's just that sometimes, when there's no sudden moves or anything, she'll just jerk back.
And that just SCARES me, it's so quick and sudden for no reason. Or sometimes, she'll just stare at me in the 'S' striking
position. Perhaps it's just the way she's shaped and she's not actually in the defensive 'S', maybe i'm just overthinking things.
Although she has never tried or anything, she just makes me a little nervous even though I had started out calm.

The day I got her, the guy at the reptile shop said it was their feeding day that day,
he had said, once I bring her home, she may not eat because of the new location and need to adapt.
I waited to days until feeding her a frozen mice that he said was what she's currently eating.
After de-thawing and all that, I dangled it in front of her, she did the 'S' position but never actually went for it.
After, I tried 'braining' as I've read it was most likely going to work... No luck.

I'm not quite sure what the next step would be or how I would go about handling and feeding her.
I'm more than willing to put in the effort in seeing her grow and be a active, social snake.
Any tips and advice would be greatly appreciated. Thanks guys.