You do not need a diagnosis to start. You need a count. For one week, keep a plain note by the bed or on your phone. Each time she stops, grabs something, sits down hard, or falls, write the time and which of the six stops on your floor plan it happened at. Write down what she said afterward, in her own words, without adding your guess.
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What the count tells you
If most of the entries land at the same stop, the trouble is in that place on the route, and that is where your first look should go. If the entries are spread across the whole trip, the problem is more likely the trip as a whole, including how long it takes and how much light she has to see by. If she has trouble at that same stop during the day too, the night is not the whole story.
Count the near-misses along with the falls. A hand that reaches for a door frame, a pause before a step, a turn that gets wider than it used to be: these are easy to miss, and they often show you the trip is starting to fail before anyone is hurt.
What to do with the count
Take the note to her physician. The note does not answer the clinical question, and it is not meant to. It gives the physician the times, the stops, and her own description, which is more useful than a summary from memory. Ask the physician whether anything on the list needs a check before you change the route.
If she is hurt, cannot get up, hits her head, or seems newly confused, do not wait for the week to end. Call for urgent help now. The count is for the slow problem, not the sudden one.