Vomiting, fever, chills. Urine examination shows pus cells. Taken Ciprofloxacin. Reason for sterile UTI?

Dear Doctor, I had severe vomiting , fever with chills on 30th of October 2012, my urine examination showed pus cells 124/ hpf. I was treated with ciprofloxacin 500 mg bid for 5 days. I was relieved of the symptoms of frequent micturation with burning sensation. I was symptomless for 20 days and on 20th November 2012 I started with chills and consulted a gp doctor and prescribed me same medicines for 3 days and requested for urine test and ultra sound of abdomen urine culture , X-ray, complete blood examination. There was no growth on urine culture, pus cells were reduced to 75/hpf, ultrasound revealed mild cystitis with normal size of prostrate and no calcification in the urinary system. Doctor what could be the reason for sterile UTI, do I need to worry about any serious condition. Thanking you