Normal Prostate Size Prostate Infection?

Prostate infection? - normal prostate size

I am 28 years, beginning six months ago in order to feel something wrong in the area between the rectum and the scrotum, the pain is very little.
A month ago I did see a doctor examine you and went everywhere, so have my ultrasound by the parties, I said that everything is working normally, was the size of the prostate g. 18 and it is normal for his age. So, you ask me to pass the urine test and said everything is in order.
Prescribed by this, for me, "Vibramycin" and said most likely you have an infection of the prostate.
After the second table, the pain is completely gone, but the use of the pill for ten days.
After more than three weeks, the same pain started to win. It is the same ....
I drive regularly, to keep myself and my own apartment, without a partner, living alone.
What should I do? What is it? I want the same medicine again?