Friday Friends: I'm Stumped

Lucy gets more than her share of attention. She demands it. She is extremely bright, catches on quickly, and loves to play.

We've had her for about 16 months - close to a year and a half. She was more difficult to house train than any of our other dogs. But then she seemed to get it. We have three doggie doors that allow her into a fenced back yard that is about half an acre in size, and she can come and go 24 hours a day.

So why has she started urinating on the carpet again?

We did not get any new pets - dogs or cats or anything else other than fish babies who stay in their tank.

Nothing traumatic has happened. No one left for college. No one got married or brought a stranger into the house. No furniture has been moved. I'm so boring I am totally predictable.

She is spayed and is perfectly healthy.

We have a collie (female) who is extremely docile. And a Jack Russell (male) she plays with constantly. Neither of them are having issues.

Any ideas on why a dog will suddenly lose their house training?