Could be a boil that needs to be drained by a professional. Is it getting bigger? Does it seem to have fluid inside? Is it discolored?

Quetzlcoatl is right. This is very likely something that needs to be drained by a professional. Odds are that if you let it sit the way it is it will harden and be difficult to remove. I'm not sure where you live, but if you're insured it shouldn't cost you a penny.

It could be a sebaceous cyst and needs to be checked by a Doctor or Dermatologist. Sometimes if those are left for too long they can grow to the size of a golf ball. They also sometimes go away on their own but, please go be seen by a doctor just to be on the safe side.

Thank you all for the advice, I will arrange to see my family doctor.