Cysts, lumps and bumps. Sometimes lumps, bumps, cysts and swellings appear under the skin. These are usually harmless, but sometimes need to be checked by a doctor. The most common cause of skin lumps and bumps include: 1  Skin cysts. 2  Cherry angioma. 3  Dermatofibromas. 4  Epidermoid cysts. 5  Folliculitis. 6  Keratoacanthoma. 7  Keratosis pilaris. 8  Lipomas.