According to your description that a “lump” left after some “spot” on the torso healed, you could be experiencing some type of atheroma. An atheroma is a retention cyst that originates from the sebaceal gland in the skin. The sebaceal gland normally excretes its sebum through a duct into the hair follicle. If that duct becomes obstructed, sebum collects inside the gland. This type of retention can be transformed into a cyst, or "atheroma". An athaeroma can become inflamed from time to time. Infections can heal spontaneously or with the use of local antibiotics... but cyst can be cured only surgically by removing the whole cyst’s membrane. If some small part of the cyst’s membrane is left inside the skin, the atheroma will reappear. You can consult a plastic surgeon for a consultation, diagnosis, and treatment plan.