UTI or bladder crystals?

Hi. I’m 29yo female. History of ulcerative colitis taking mesalazine, budesonide, vedolizumab. For a year I’ve been experiencing severe uti episodes. Symptoms include urinating visible blood and severe burning and cramping pain. Always test negative on cultures. Antibiotics don’t seem to help much. I’ve tried approx 6 diff kinds. Ultrasound, CT w contrast and Cystoscopy normal. Because I’ve had a few dips lightly positive for nitrites doctors say uti. I wonder though if it’s urine crystals. During these flare ups I pee out what doctors say is my bladder lining but I’m not sure. It clumps together into a grainy substance and crucially under a light it shimmers? I’d appreciate any thoughts here, thanks.