Causes can be divided according to the affected organ and they are
Kidney conditions (infection, pyelonephritis,
kidney stones, polycystic
kidney disease), Bladder conditions (cystitis, bladder polyp, bladder tumor), Urethral conditions (Sexually transmitted diseases, urethral injury, vaginal inflammation), and certain bleeding disorders. You should visit your gynecologist who needs to conduct a detailed examination to rule out an STD and find the exact cause.
Hope it helps.