One of my two cats has suddenly started to defecate indoors .
Both of the cats have always been well trained and very clean and use a cat
door to go outside to "do their business"
Out of the blue  the female went into the corner of my sitting room and
defecated on the carpet.
We decided to keep both cats in the utility room for a few days where they
are normally kept at night....with the cat door access to the garden.
After 3 days we let them roam around the house again....and she went back to
the same spot and fouled it again.

Has anyone got any ideas why after 12 years of spotless behaviour the cat
has now done this twice ?

Any advice or insights gratefully received

many thanks

Barry