Bacteria getting into your urethra.
This can happen by wearing thong knickers, tight/synthetic fabrics, wiping from back to front when you go potty (always wipe from front to back other wise you wipe botty bacteria straight up to your vagina and urethra).
Also: not empting the bladder properly - there can be bacteria in the urine and if you don't empty the sit in there and multiply, friction across your urethra during sex can push bacteria in as well.
However the most common cause is wiping the wrong way.