Prostatitis & burning pee sensation.

I suffered from several urinary and sexual symptoms (e.g. burning urine, frequent urination, erectile dysfunction, delayed ejaculation) for a month or two untill I decided to go to the a nearby medical center. Through ultrasound I found out that I have prostatitis and the seminal vesicle next to it appeared to be inflamed as well. I did multiple blood and urine tests for a variety of infections even STIs (including urine analysis and culture) but all results were negative. I finally decided to do a urethral swab (sample & culture) & answer was: [COAGULASE NEGATIVE STAPHYLOCOCCI IS ISOLATED IN HEAVY GROWTTH). We found out that it's resistant to seven antibiotics, and is only sensitive to four. Out of the 4, I decided to get 1-time IV of ciprofloxacin. Starting the next day, I took Levofloxacin500mg daily for 10 days. Last day taking the antibiotic was Wed or Thurs. ALL of my symptoms while on treatment disappeared completely. HOWEVER, starting from Sat, burning sensation returned. Why?