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To get an accurate diagnosis you need to be seen by a doctor. It could be a number of things such as an STD (if you are sexually active) or something as minor as an ingrown hair, but only your doctor can give you a proper diagnosis and treatment.
It sounds to me like it might be a boil. Here is some info on how you can treat it : How to Care for a Boil | eHow.com. You might as well start somewhere. If this doesnt help, I would see a doctor