Erythema is a skin concern characterised by redness or rash. There are many types of erythema, including, photosensitivity, erythema multiforme, and erythema nodusum. Photosensitivity is caused by a reaction to sunlight and tends to occur when something, such as an infection or medication, increases your sensitivity to ultravoilet radiation.
Erythema multiforme is characterised by raised spots or other lesions on the skin. It is usually caused by a reaction to medications, infections (especially herpes, simple virus), or illness.
Erythema nodosum is a form of erythema that is accompanied by tender lumps, usually on the legs below the knees, and may be caused by certain medications or diseases.