Got a bump on my penis. Only one. Rather large, originally thought it was just a pimple. Noticed it about 36 hours ago. Hasnt changed size but it has turned black in the center when it was originally red and opaque. What could it be and what is the best course of action from here on out?

It's painful. Especially on contact. Pain level low. First time ever seeing something like this. Yes I'm sexually active but have Had the same sexual partner for the past 1.5years.