Cellulitis: This is a common skin infection that can cause unilateral erythema (redness) and tight edema (swelling). It typically presents with warmth, redness, and swelling of the affected area, often accompanied by fever and malaise.

Deep Vein Thrombosis (DVT): DVT can cause unilateral swelling and erythema, especially in the lower extremities. It is often associated with pain and warmth in the affected limb.
Contact Dermatitis: This is a type of skin inflammation that can occur after exposure to an allergen or irritant, leading to unilateral erythema and edema.
Insect Bite or Sting: Certain insect bites or stings can cause localized erythema and edema, which can be unilateral.

Necrotizing Fasciitis: Although rare, this is a life-threatening condition that requires immediate attention. It can present with unilateral erythema and edema, but is often accompanied by severe pain, fever, and crepitus (a crunching feeling under the skin).
Compartment Syndrome: This is a serious condition that occurs when pressure within a muscle compartment increases, leading to nerve and muscle damage. It can cause unilateral swelling, erythema, and pain, especially in the affected limb.

Erythema Elevatum Diutinum: A rare skin condition characterized by persistent erythema and edema, often on the extensor surfaces of the limbs.
Chronic Lymphedema: A condition characterized by chronic swelling of a limb due to impaired lymphatic drainage, which can cause unilateral erythema and edema.
Lipodermatosclerosis: A rare condition that causes inflammation and scarring of the subcutaneous fat, leading to unilateral erythema and edema, often in the lower extremities.