Pls define erythematous nodumus. is this a rare disease?

"red lumps" [ Greek]: Erythema nodosum (en) (red nodules) is an inflammation under the skin characterized by tender red nodules or lumps. It does not represent one specific disease, but rather one outward manifestation of any of a number of different infectious, allergy, immunologic, or unknown-caused situations. http://www.medicinenet.com/erythema_nodosum/article.htm.

Red tender bumps: Under the skin. Erythema nodosum presents as painful, tender, red bumps under the skin one the shins and elsewhere. It may be associated with other disorders but about half the time the cause is not known. While uncomfortable the lesions are not dangerous. Please see this site for more info. http://www.ncbi.nlm.nih.gov/pubmedhealth/pmh0001884/.