Well it just had happened earlier today. The only time it happened so far.
To look for a STD you must have a specific STD test.
From my understanding, you were diagnosed with Urinary Tract Infection rather STDs like Gonorrhea and Clamydia right?
Testing for white blood cells can tell the level of bacteria in urine but it's not a test for Gonorrhea and Clamydia.
I assume your doctor never suspected about Gonorrhea or Chlamydia. Normal level of white blood cells in your urine indicates that the bacteria in your urine is now at the normal level, which is a good news.
How often do you get discharge? You should talk and show your doctor about this.