The problem is that she has been spayed. I understand from reliable internet sources that these things happen sometimes. A little left over ovarian tissue is all it takes.
At first we thought that she was just being more cuddly than usual. Then Stubble noticed the "butt up, tail in the air" pose. Then we all noticed the "vocalization". Last night she marked Stubble's bathroom by the open window to let the neighborhood Toms know that she is both available and willing...
Tomorrow she has an appointment at Prod-Your-Pet Veterinary Hospital for evaluation and (probably) additional surgery.
Poor little Katsu!