Thanks for writing your query.
It will be helpful if you give more information about any medicine she is taking.
After reading your post, it seems that your mother is having postural edema in her feet.
It occurs due to standing or sitting for long durations and decreases gradually on elevating the legs.
Still you should get some investigations like :-
4. Serum Protein levels.
5. Urine examination.
Treatment can be planned according to the reports of the investigations.
I hope this is helpful to you.