What Causes Sore Penis?

I recently shaved my private area. I then was masturbating and noticed on my left side of my shaft, it was sore. I let it go for a couple days to recover. Then I went back at it. I woke up this morning with a bunch of pimple like sores in a cluster around the area that was previously irritated. I am not popping them. My penis is sore, there. What is it? I was using Vaseline... Do I have some sort of bacteria skin infection? What should I do?