What Causes Red Spot On Penis Head?

Hi about two and a half weeks ago I noticed a red spot on the head of my penis. It was a little smaller than a dime. It was flat. It did not itch or cause any pain. The redness started going away after a week and got much smaller. The only medication I was taking was a prescription medicine for my cold sores. Now the red is all gone. But today, my feel a little painful testicles. I checked and it feel the left vein is swollen and it hurts. I did recently have a vasectomy about 3 months ago. Could this be an std?