Erythema nodosum is a type of panniculitis. Panniculitis occurs when there is inflammation of subcutaneous fat (the layer of fat lying underneath the skin). The inflammation causes rounded, tender red bumps (nodules) to form just below the skin surface. Erythema nodosum most commonly affects both shins.
This is a companion discussion topic for the original article at https://patient.info/skin-conditions/erythema-nodosum