Erythema Nodosum:

Erythema nodosum is a skin condition that causes painful, red or blue lumps or nodules to appear on the skin. The NHS website explains that erythema nodosum usually goes away on its own, but it can sometimes be a sign of a more serious condition. 

Erythema nodosum

Symptoms

Painful, red or blue lumps or nodules, usually on the shins, thighs, or arms

Other symptoms

Low-grade fever, malaise, fatigue, swollen lymph nodes, cough, joint pain, weight loss, stomach pain, vomiting, or diarrhea

Causes

Bacterial throat infections, certain drugs, inflammatory bowel disease (IBD), tuberculosis, Hodgkin's disease

Treatment

Medications, bed rest, treatment for any underlying condition

Duration

Symptoms most often go away within about 6 weeks, but may return