Contact Dermatitis: This is the most likely diagnosis given the recent changes in body wash and detergent. Contact dermatitis is a type of inflammation of the skin that occurs when the skin comes into contact with an irritant or allergen, which could be a new ingredient in the body wash or detergent.

Irritant Contact Dermatitis: Similar to contact dermatitis but specifically caused by an irritant rather than an allergen. The change in body wash or detergent could introduce an irritant to the skin.
Atopic Dermatitis (Eczema): Although less directly linked to the change in body products, atopic dermatitis can be exacerbated by irritants and could present with a rash on one extremity.
Psoriasis: An autoimmune condition that can cause skin rashes. While not directly related to body wash or detergent changes, it's a common skin condition that could present with a rash on the lower extremity.

Cellulitis: A bacterial infection of the skin that can present with redness, swelling, and warmth of the skin. It's crucial to differentiate from other causes of rash due to its need for antibiotic treatment.
Deep Vein Thrombosis (DVT): Although DVT typically presents with swelling and pain, in some cases, it can cause skin changes that might be mistaken for a rash. Given the potential severity of DVT, it's essential to consider it in the differential diagnosis.

Erythema Nodosum: A condition characterized by the sudden formation of painful, red lumps on the skin, often on the lower extremities. It can be associated with various conditions, including infections and autoimmune disorders.
Lichen Planus: An inflammatory condition that affects mucous membranes inside the mouth and can also affect the skin, presenting with purplish, itchy, flat bumps. It's less common but could be considered if other diagnoses are ruled out.