Allergies: This is often the most common cause of redness under the eyes, due to allergic reactions which can cause blood vessels to become visible and give the appearance of redness.

Fatigue: Lack of sleep or excessive tiredness can lead to puffiness and dark circles under the eyes, which may appear red due to increased blood flow.
Dehydration: Not drinking enough water can cause the skin under the eyes to become dry and irritated, leading to a red appearance.
Sinus Pressure: Congestion in the sinuses can cause the blood vessels under the eyes to become swollen and red.

Cellulitis: A bacterial infection of the skin that can cause redness, swelling, and warmth, and if left untreated, can lead to serious complications.
Angioedema: A condition characterized by the rapid swelling of the skin and mucous membranes, which can be life-threatening if it affects the airways.
Basal Cell Carcinoma: A type of skin cancer that can appear as a red, shiny bump under the eyes, and if not treated, can lead to disfigurement and potentially spread to other areas.

Erythema Multiforme: A skin condition characterized by target-shaped lesions, which can appear under the eyes and is often triggered by infections or medications.
Contact Dermatitis: A type of skin inflammation caused by contact with irritants or allergens, which can cause redness and itching under the eyes.
Ocular Rosacea: A condition that causes inflammation of the eyes and can lead to redness, itching, and burning sensations under the eyes.