Prostatitis.

Unfortunately or fortunately, depending on how you look at it,
bacterial prostatitis is rare.
My guess is they did not isolate any pathogen from urine or elsewhere.
The antibiotics will hopefully work although antibiotics themeselves
have anti-inflammatory and immunomodulating properties on their own.
They may provide symptomatic relief or resolution even when the origins
of the prostatitis is in doubt.
There is an NG here on usenet and websites out there that cover the
topic. Just hope that the antibiotics work and you won't have another
episode down the road, something common for prostatitits sufferers.