What's one word for the phrase 'the act of taking your head back on seeing something strange'?

This is not a single word, but it works for your purposes: taken aback

This can be used in the figurative sense (and usually is), meaning:

(MW.com)

to make a strong impression on (someone) with something unexpected

It's origin is a nautical term:

(Etymonline)

Now surviving mainly in taken aback, which originally was a nautical expression in reference to a vessel's square sails when a sudden change of wind flattens them back against the masts and stops the forward motion (1754).

Literally, it means being pushed backward (as the head in your example) or halted in your tracks.

Start, perhaps - To (cause to) make a sudden movement, and related senses.(OED)

To make a sudden movement, esp. of part of one's body, as to avoid a blow or perceived threat; to flinch or recoil from something in alarm or repugnance. Chiefly with from or with adverbs (as aback, aside, away, back, etc.). Also with the part of the body as subject. (OED)

To undergo a sudden involuntary movement of the body, resulting from surprise, fright, sudden pain, etc.; (sometimes without implication of actual movement) to feel startled or momentarily perturbed, as at a sudden realization. (OED)

So,... "She turned away and started at the sight of the filled porcelain."

From the full (subscription only) Oxford English Dictionary...

shy intransitive
1: to take a sudden fright or aversion;
to make a difficulty, ‘boggle’ about doing something;
to recoil.
2: (Of a horse) To shrink or start back or aside through sudden fear.

When to shy is used with a person as the subject, it's usually in more metaphoric negating contexts (He never shies away from hard work). But synonyms such as recoil, shrink - and passive be repulsed / repelled [by] - can all be used to convey rapidly jerk back from [something strange / disgusting / scary], if you don't want to potentially liken your subject to a skittish horse.

I'd also say recoil or flinch:

recoil: to make a sudden movement away from something esp. because of fear or disgust

flinch: to withdraw or shrink from or as if from pain

I think Killing Time's comment fits well, too:

She turned away and did a double take at the sight of the filled porcelain.

A "double take" involves a bit of a delayed reaction, which I envision as having a physical component. Imagine the situation Merriam-Webster gives as an example:

His parents did a double take when he came home with a tattoo.

I imagine that they say "hi" as always and are already turning back to whatever they were doing before he entered, only to turn their heads back, startled, because by then the realization had surfaced that he has this new image on his skin that wasn't there before.

The movement may well involve moving the head backwards as when you try to assess something skeptically, frowning, symbolically increasing the distance to the subject of scrutiny, thinking "wait a minute". You can surely do a double take that way and still look away, if you want to avoid eye contact or don't want to stare; in that case you would freeze for a second, perhaps mentally "replaying" what you saw and considering a proper reaction, without taking a closer look right away.