Hello, Anon. Thank you for posting.
Do mean a cyst? A cyst is a closed capsule or sac-like structure, usually filled with liquid, semisolid, or gaseous material. Cysts usually occur within almost any type of the body's tissue and vary in size.
If it is a cyst, keep it clean by washing with warm soap and water. It might be tempting to squeeze it, but doing so might cause infection. Try putting a warm (not hot) compress on it for 20 or so minutes, a few times a day, to help soothe it and speed healing. Wrap it in a bandage if it starts to drain. An antibiotic ointment can be used if you think it has become infected. Make an appointment with your doctor so it can be examined if it does not go away or worsens. The same would be true of a boil which is a painful, pus-filled bump under the skin caused by infected, inflamed hair follicles.