What Causes Itching On Penis Along With Burning Sensation During Urination?

Hi, I'm about to deploy on a ship. The last three days, I've had some burning when I urinate, and my penis feels itchy when I pinch it. I tried to see my doc today, but could not. I'm sure I have an STD, and wondering if cephalexin 500mg will be helpful. I have about 15 pills from a RX dated this past December, so they are fresh.

Thanks

Joe Bloom