I would go to your doctor or someone at a clinic like Planned Parenthood to have it checked out. It may be linked to an STD or possibly a tumor. If it turns out to be nothing, at least you'll have that peace of mind.
First check to make sure that it's not an ingrown hair. Ingrown hairs can cause rather sizable lumps and, similar to a pimple, can cause pus to form inside. If this is the case, follow procedures dealing will ingrown hairs. If not, one should get it examined by a doctor, especially if there is a history of sexually activity.
Although the most likely explanation is that there is nothing wrong and it is just caused by natural variations in your body, you should still be sure to get it checked out by a doctor. It is pretty unlikely you have cancer, but do you want to take that risk?