25 y.O.Female annual urine test shows microhematuria. No pain, no symptoms, and no infection. Lab results say no abnormal cells. What's the cause?

Difficult to say. A work-up is recommended. A significant percentage of patients in your age group have a normal result on the tests and therefore no apparent cause. The first thing to do, in my opinion, would be to simply repeat the urinanalysis. Kidney stones are a possibility. Make sure the test wasn't done around the time of your period. If it was, the blood may have not been from your urinary system at all.