Why is my grandmother experiencing severe short term memory loss when a stroke was ruled out?

After being in hospital due to a reaction to a diuretic(hyponatremia and hypokalemia), which was resolved and grandma was back to prior level(lived completely independent,no signs of dementia), two days later she displayed severe short term memory loss. Perseverates on same 4-5 questions, will ask the same question or make the same statement every 1-2 minutes. An MRI ruled out a stroke, any other explanations? She is on very little medications.