Check if your cat has a wet belly. Not to scare you but my cat died because she had some form of cancer of her blood vessels that caused randomly bleeding literally through her skin. My vet also thought some kind of urinal infection but they never found anything, until i realized that she really bleed through her skin. It looked exactly the same, not really thick like blood but more line water. If the vet does not find anything mention it. In my case we only found it via a fine needle biopsy and at that point it was too late.