Recurring blood/clot after testing twice

My cat one every three to four months cries really bad, tries to pee, drops several drops of blood then a tiny clot and she's better?! The whole process last about 20 min.
Twice I've paid $300 for visit and bloodwork and all levels look fine. The clot is much tinier than a raisin but looks similar. Picture attached.
I've read apple cider vinegar may help?? Suggestions? Opinions,. Once again she's fine now that it passed.. But what is it??

Do a search for FLUTD here and read a lot of what you find in those threads.  It sounds to me like she is getting struvite crystals which irritate her urinary tract and bladder like tiny shards of glass, and when she passes them, she is better.  The Braggs apple cider vinegar is good for keeping the urine slightly acidic.  If you can keep her urine pH at about 6.0, she cannot form the crystals.  I have a cat like this.  She gets just 0.2 mlof the vinegar mixed with 0.8 ml of water once a day as maintenance.  Back when she was having attacks and I was working harder to keep the pH below 7.0, sometimes I would do it up to four times an hour apart before the pH would come down.  I also feed her totally grain free canned food, no dry food at all (Abound grain free chicken, from the Kroger group of stores).

I bought pH strips from eBay and tested her urine frequently until I got to the right balance.  She tended to have a slightly alkaline urine (7.0 to 7.5) and crystals could form, but they can't form in a pH of 6.0 to 6.5.  I also got L-methionine, a natural amino acid supplement, and add a little bit to her food (a pinch, like if it were salt and mix it in, she likes the taste actually)  as that also causes the pH to be slightly acid.  This same regimen is working on a friend's male cat who had been completely blocked and had been flushed out twice at the vet.  It seems to be saving his life.  A clot like your female passed could actually block a male's urethra, and that could be fatal if it wasn't treated.  My friend's rescued male has been on this regimen for a month now and has not had to see the vet again.