So, when you think of shapeshifting, what do you imagine?
In this world there are two types of shapeshifters, out of which Nymeria is the second kind.
The first one isn't actually physically shifting, but rather illusion magic making the holder appear else. This kind is more comfortable and safe than the other type, but has certain issues: your apparition cannot separate from you, meaning even if you appear to be a bird you cannot fly freely, and if you are tied up your apparition is useless in freeing you. Also, even if you appear else, your original physical form is still… physical. And the apparition is not. Thus if you appear as something small, something might hit your actual body, and if you appear to be larger than your original shape, well, your elefant trunk can't lift or touch anything, and you certainly can't drink with it. However this kind is perfect with mimicking the appearance of another person.
The second kind of shapeshifting is Nymeria and her relatives 'gift'. It is physical shifting, but it's slower and often quite painful: body manipulation. Forcing bones and skin to break and reshape themselves into something new.
Nymeria is the holder of the third generation of this gift, so it is restricted. She can control hair, nails skin, muscles and bones, but has never had much practice with regrowing organs or more unique bodily features.
Nymerias mother was dead before she realized her gift, so she was never properly taught the rules of the art- thus she came up with them herself:
Necessary bodily functions cannot be compromised- ever. Nymeria could regrow some things, such as limbs, but no organs. Thus, she can never shift into something smaller than the mass of her organs.
Of course Nymeria has made mistakes against this rule. She has several times had to patch up lungs to the best of her abilities and one of her eyes has bad sight after being exposed to too much pressure.
The other rule is that physics still apply. Even if she could grow wings out of her back, she wouldn't be able to fly unless they were proportional and her body had the right sort of muscles to use and hold such wings up. She could of course turn into something that didn't abide by physics, but her movements might be utterly compromised.