During winter due to cold
and nose or ear fullness, vestibular apparatus will be affected which can lead to vertigo
or giddiness which comes on change in position.
I think your mother might have same problem which is causing giddiness.
she needs rest, drugs to suppress cold and antivertigo drugs.
if vertigo persists even after treatment with above drugs and it is increasing, you have to consult neurologist
and take his opinion.
I hope you understood, get well soon.