It's exactly what Tigris has- unknown reason.

The reasons could be viruses that are not identified yet or an autoimmune process. If you search for iFLUTD in Google you will find links.

In top of that it is easy to get a bacterial superinfection. (Super meaning "on top of it"). So you will also find UTI bacteria. This is why usually antibiotics are given to kill them.

In the case of iFLUTD you don't have crystals. Thus changing of the food is not needed as all the UTI food prevents the forming of crystals.

What you do is trying to keep the inflammation down whose cause is not known. Usually this is done by cortisone. In addition a lot of drinking is necessary so that the urinary tract is rinsed properly.

In the case of Tigris this worked out nicely. Our vet informed us still that it may happen the problem comes back. Then we would try the same medication. She says she knows cats who have this problem once a year but are fine the other times.

Tigris is now ok for 8 months. I hope it'll stay like that as long as possible and wish Sunny the same


Edit: I add a text from a website about the iFLUTD:

"Idiopathic Lower Urinary Tract Disease
Although cats with lower urinary tract disease behave in similar ways, the potential causes are multiple. Urinary tract infections (bacterial, fungal, parasitic, and perhaps viral), urinary stones, urethral plugs, cancer, and other disorders can affect the lower urinary tract of the cat. Unfortunately, in spite of extensive diagnostic tests, the cause of over half of the cases of feline lower urinary tract disease remains elusive; such disorders are called idiopathic feline lower urinary tract disease (IFLUTD). Cats suffering from IFLUTD make frequent attempts to urinate, probably as a result of bladder discomfort, and often are found to have blood in their urine. Dietary management (see below) has reduced the likelihood that cats with IFLUTD will develop a urethral obstruction, but there is no evidence that these "special" diets have reduced the incidence of idiopathic feline lower urinary tract disease itself.

Veterinarians have recently noted many similarities between this common form of feline lower urinary tract disease and a bladder disorder affecting humans called interstitial cystitis (IC). A psychologically stressful event often precedes the onset of lower urinary tract discomfort due to interstitial cystitis in humans. Interestingly, in one study, a recent weather change or a move to a new environment—both potentially stressful events to a cat—were factors related to the onset of signs related to IFLUTD.

A number of therapeutic methods have been attempted, but none are uniformly successful in the treatment of either IC in humans or IFLUTD in cats. Studies are ongoing to determine whether the human and the feline disorder are truly the same, and whether therapies helpful for humans will be of benefit to cats as well. Thankfully, most cases of IFLUTD resolve within a short period of time, even without treatment."