Is my ball python underweight?

She has a hide, and a water dish.

Stop handling her and leave her Alone for 7 days. she's probably very, very stressed out.

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Your snake is stressed out. Do not handle her until she has eaten. And since she seems thin, do not handle her until she has eaten 3 consecutive meals. The reason she won't eat is because you're messing with her too much. You should have left her alone for about a week before handling/offering food. Its a general rule of thumb to give new arrivals at least a week to de-stress and adjust to the new environment. (This means no feeding or handling. Just spot cleaning)
Some snakes will take food food within the adjustment period and some won't.

And a mouse fuzzy is too small. Offer a live mouse Hopper or live fuzzy rat in a week. And again, do not handle or mess with your snake until it has eaten a few times. Assist feeding is also highly stressful and should not be attempted unless it's a last resort.
I don't think your snake is at the point yet.

I also suggesting adding a secondary hide. Or darkening the enclosure with paper or something.

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Yep, slow down a little and take the advice of the (other) folks around here and your snake will be fat, happy and hidden.

Alright I'll leave her alone for awhile. She is my first snake (Besides the wild caught California Kings) I have ever had.

I noticed something strange last night, I looked a her belly and some of it is sunkin in, is that normal? Like around her neck. It got me really worried.