Why does my 5 year old female cat keep having this recurrent problem where her urine has blood in it?

some background info: about 2½ years ago this cat had bladder stones. she was peeing all over, and the urine was pink with blood. I got the operation done, which was about $1,000.

About two months ago, she had the same problem. i took her to the vet and he did an xray, and said she had blood clots in her bladder, which was from friction (maybe she had crystals in her urine). he prescribed antibiotics and a steroid. problem cleared up and i took her for a follow up visit. he tested her and said the urine was negative for infection. that was about a month ago.

now, she has the problem again. i am wondering why she could possibly have this problem? she does not have stones, because if she did, they would have showed up on the xray two months ago. she’s not old – she’s only about 5. other than this problem, peeing bloody (mostly she pees in the tub so i don’t mind that) she is acting normally, eating, everything else cats do.