This is a difficult question to answer, as I cannot examine your mother or see the actual lab results that you talk about in your question. That being said, the
symptoms you describe your mother having can be brought on by low sodium. For the most part, low potassium and low sodium are usually caused by kidney problems, malabsorption syndromes, diabetes, or medications. I would take your mother to see her primary care provider and ask them to look into this problem, to see if can be fixed. They should review her medications, as these may contribute to the problem. She may also have a kidney disorder or an endocrine disorder, like Addison's disease or aldosteronism, but these disorders can be found with simple blood tests.
When she goes to see her provider, please bring a list of all medications she takes, including over-the-counter medications, vitamins, or any herbal supplements she is using. Also, pay attention to her diet. Is she watching her salt intake? Does she eat licorice? Is she taking alot of laxatives? Let her provider know if you have any additional information. Good luck!