What Causes Bump In The Genital Area Post Unprotected Sex?

I have a question about a genital bump and whether or not I should worry. I have only had unprotected sex once in the past 7 or 8 months and it was 2 weeks ago. Two days ago, I noticed a small, round painful red bump at the base of my shaft. I want to know if this is a common issue in that area or if I should be concerned about STDs.