can be a boil/furuncle, carbuncle or infected lipoma or fibroma. The exact diagnosis is possible clinically on actual physical examination. Do you have any associated discharge or pus or any other symptoms?
Boil and carbuncle are skin conditions caused by the infection of hair follicles resulting in the localized accumulation of pus and dead tissue. The symptoms of boils are red pus-filled lumps that are tender, warm, and painful. They usually caused by staphylococcus aureus infection. However, they usually resolve on their own or after a short antibiotic treatment.
You should consult a dermatologist for a complete evaluation of the swelling as it is present since long time.