You might want to be careful. I don't want to scare you, but do you have any possible stds, or any possibility of getting any? That's how a lot of them start out. It gets very sore and then more defined. Course if you have an infection it could do the same thing. I would get it checked out.
Even though niceguy was alittle out of his place...He is right. I would take a closer look. Maybe it got rubbed raw after having intercourse, but you want to be careful.
It may also be just an ingrown hair. Keep the area clean, and if it persists another day or two, get it looked at, just to make sure its not an std or anything. Or maybe tell your partner to take it easy next time it could very well just be the result of a little roughness!