What Causes Reddish Bumps On My Penis?

After vigorous masturbation without a lubricant (through the material in my shorts), i have now noticed three small red bumps (one with a slightly white head) on my shaft, just under the ridge of my penis... I am always paranoid about herpes. I had blood tests ran several months ago and they were negative. I had the recent/past exposure tests ran a couple of weeks after a risky encounter and they were negative. I have had the same partner since then. I also had a full STD screen around the same time as the herpes blood tests. What can the bumps be? they have been there for about five days. What can i do to get rid of them? they are not painful, just a bit itchy at times.