Has your vet talked with you about FIC (Feline Idiopathic Cystitis)?
If not, there is a very informative website that describes this disease that I want you to take a look at:
About 65 percent of cats suffering from cystitis have a condition called idiopathic cystitis, with changes to the lining of the bladder. The bladder lining is a thin film of protective mucus (glycosaminoglycan, or GAG) that prevents microbes or crystals from sticking to the bladder wall.
Sand (crystals) or stones (uroliths) in the bladder account for about 10 percent of urinary tract obstructions. In 60 percent of cats, material leaks from the inflamed bladder wall, producing a substance that can accumulate in the urethra, forming a urethral plug.
If a cat is also producing some urinary crystals, these accumulate in the plug, making it even more likely to cause a full obstruction.
Finding a few crystals in urine is not significant on its own unless there is associated disease or a history of previous disease. Equally, finding no crystals does not rule out a stone in the bladder. Some stones shed no crystals.
Let me know if you have more questions.
You might also want to print the article mentioned above so that you can take it with you to your next appointment.