What might be wrong with an older woman that has forgotten half of a day?

My grandmother was taken to the hospital yesterday. My younger sister, who lives with her, said on her drive home yesterday, my grandmother was telling her to "pick up the shoes". My sister didn't know what she was referring to, and there were no shoes in the car. And then my mother says that she can't remember what went on yesterday at all after a certain time. My mother went with her to the hospital, and says that all the test they've run so far, cat scans, bloodwork, etc., have all come back normal. But she's running a high fever. What could be wrong with her? And don't tell me to go see a doctor. The doctor doesn't have any clue yet.