Dermatofibromas are small red-to-brown bumps (nodules) that result from an accumulation of collagen, which is a protein made by the cells (fibroblasts) that populate the soft tissue under the skin.
Dermatofibromas are common and usually appear as single firm bumps, often on the legs, and particularly in women. Some people develop many dermatofibromas. The cause is unknown. Dermatofibromas are harmless and usually do not cause any symptoms, except for occasional pain or itching. Usually, dermatofibromas are not treated unless they become bothersome or enlarge. A doctor can remove them with a scalpel.
Resource: Merck Manual Home Edition