I have traces of blood in my urine , was tested for kidney stones and nothing ! what could it be ?

Micro-hematuria : The workout for microhematuria includes upper tract imaging such a ct scan as well as cystourethroscopy. Urine culture and urine cytology should also be performed.

Many things....: "blood" is often recognized in the "chemical" portion of a ua, but may represent one of several different proteins in the urine and not truly blood or hemoglobin. Rbcs in the microscopic ua confirms blood. Could be stones/uti/cancer (tcca/rca/sqcca/...). Needs full evaluation with microscopic ua (repeated) and with ct ivp/cytology/possible cystoscopy.