Hello XXXXXXXX;
A couple things come to mind.
Is this a new home or did some one else live there before? There are often scents that cats can smell that we can not. It is possible she is marking her new home with her scent.
Anxiety may be part of the issue. It has recently been shown that anxiety can set off cystitis like conditions in some cats.
I assume that a complete urinalysis was done checking her for crystaluria.
I she checks out completely well and no crystaluria then a combination of behavior modifying drugs like Prozac or xanax and cleaning the house with a urine odor eliminator may help or cure the problem.
Just a point of interest you may want to purchase a black light from a local pet shop. Feline urine will fluoresce under it. Make sure if you treat with a urine odor neutralizer all spots are cleaned and treated.
Good Luck
Dr Dan
Cat Veterinarian
DVM, Practice owner since 1988, Memeber AAFP,AAHA, AVMA