All good advice given above. House cats are especially picky about their environment. Other than the open windows, have you changed her environment at all? Moved any furniture? Moved her food bowls or the litter box? We had a cat years ago who liked to nap on a certain old loveseat we had in our bedroom. My wife moved the loveseat into the barn as it was getting pretty funky. A few days afterwards, the cat became physically depressed. Lethargic, rough coat, wouldn't eat. After testing fine, our vet asked about her environment. When we moved the loveseat back, she quickly recovered.
My first thought though on reading your post was UTI. I'll be she's got urinary or diabetes issues. Is she drinking excessively?

Wonder if she is marking her territory with the birds.
I have a Dog and the local Cats were marking the porch on occasion.

Ant that right there my friends, may possibly be the cause of a UTI.

Cats do their business outside the litter box for a variety of reasons. It could be that the window hasn't been opened for fresh air, or that the birdies aren't there for 'entertainment' any more. But this is the only way cats really have to communicate that either they are upset or physically sick and hurting.

You said that the vet didn't catch anything during a routine check-up, but UTI or allergies or even diabeties are not exactly routine check-up sorts of things.

I'll simply concur that kitty needs to go back to the vet and let them check her out thouroughly. It may simply be a couple days of meds and a diet change that will keep your cat happy and healthy for many years to come.

. . . or the cat is just screwin' with you.

Cats also get feline lukemia and feline AIDS. I've had cats die of each in the past.

Get the cat to the vet

cat thing
 

We had to add an other new litter box in a different location, worked, We have had cats for over 30 years, the kids kept bringing them home,, now the kids have moved out over 7 years ago and the cats are still here :}
Here's a vid of my oops the kids cat,,, at 20 years old,,,check out my MB vids if you like,,
Cat sings in protest to bad player. - YouTube

Is it possible that your wife recently cleaned the litter-box with something that smells strongly?

To get the smell of the couch you can use a urine neutralizer like "Urine off" .
Cats often use the same spot again if it still smells.

Rob

Ok follow-up:..We took kitty into the Vet yesterday. Vet performed visual/hand probe exam..found nothing.
She also did a blood draw that includes complete work-up for just about any and every ailment that can befall a Feline.
Will get the results Saturday.

Hope it turns out well.

- Peter.

Oddly I went through this same senario with my now 11 yo cat last fall. About $900 later worth of test and treatment a $12 Rx for Flagyl brought everything back to normal.

Animals belong outside. They are OK with it too- it's where they came from.

Women should be able to walk around topless. I am ok with that too. Its only natrural.


'Cept in both cases (yours and mine) it doesnt apply everywhere.

Actually, where I'm from some of them did exactly that. Loses it's appeal real quick...

- Peter.

Does Singapore have any street/feral cats? I bet they do.

My guess is that your cat saw a feline at large near your home. Nothing will freak a cat out like a feline intruder.

Singapore is what can best be classified as an 'Urban Jungle'.
The area I am located in is as close to what could be called 'Rural'.
Meaning, there are a variety of wild animals lurking about.
I have ran across 4-8 ft Python's, Lizards that reach 5 ft in length, Monkeys that sit on branches and sometimes pee on passer-by-as well as come into the flats and reak havoc.
Yes there are feral cats. Some of them look quite knarly- huge visable scars/ears/tails/eyes missing etc.
We had one feral cat that would actually wander around near where we live, but havent seen this cat for awhile.
The other feral's pretty much stay away from the housing units.

With all of the above described wildlife in the vicinity, it doesnt take much imagination to understand why we keep our cat indoors.:)