Something in her life has changed and she no longer wants to use the litter box. It may take a while to figure out the cause. Meantime, try everything.

More love and attention, that always helps. Make sure that the box is well cleaned and fresh. Nothing scary or new in the area of the box.

Has there been any change in diet? Notice that the second offence was close to the first. She now knows that this does get your attention. Make no issue of it if it re-occurs. Never let them have a way to "pull your chain".

I take it that the stools were well formed and normal looking. If so you might be able to avoid a trip to the vet.