I Am Female, 72 Years Of Age In What I

I am female, 72 years of age in what I thought was excellent health. Only hospitalized once, child birth. I recently had what was thought to be a bladder infection because urination felt different and still the urge to urinate even after urination. The nurse practitioner found blood in urine at 3% -- culture was negative. I went back today and blood in urine was 10%. I have had no issues related to the bladder infection since taking a round of antibiotics. I have no pain or other issues at this time. I have never had any urinary problems before now. What could be causing the blood in the urine?