Looks like a boil or pimple. Could have been caused due to local friction , heat and many other causes . And usually it goes away without any intervention necessary but since you say it's been there for months , it might benefit from an ointment and a doctors inspection . Do meet a gp or a dermatologist and they can certainly help you out.
Content on HealthUnlocked does not replace the relationship between you and doctors or other healthcare professionals nor the advice you receive from them.
Never delay seeking advice or dialling emergency services because of something that you have read on HealthUnlocked.