Any lump should be examined by a doctor. The cause can be allergic, inflammatory, infectious and very rarely malignant. The following medical conditions are some of the causes of any lump like sebaceous cyst, dermoid cyst, neurofibromatosis or any malignancy .A dermoid cyst is a benign cystic tumor often containing skin, hair, and other tissues. A sebaceous cyst is a cyst which forms in the epidermis, is noncancerous, and is generally not a serious condition. The exact cause can be determined only after a clinical examination. This is merely an advice and not a substitute for clinical examination .Do keep me posted on how you are doing. Hope it helped.
Best wishes and regards!