Hot Shower Syndrome: This condition is characterized by the appearance of macular erythematous patches after exposure to warm or hot water, which resolve spontaneously. It's a benign condition often related to the sudden change in temperature and is more common in individuals with sensitive skin.

Contact Dermatitis: Although typically associated with exposure to allergens or irritants, some forms of contact dermatitis could potentially be exacerbated or triggered by the warmth and moisture of a shower, leading to erythematous patches.
Polymorphic Light Eruption (PMLE): While PMLE is primarily associated with sunlight exposure, some individuals might experience similar reactions to other forms of heat or light, potentially including the warmth of a shower.
Urticaria: This could be considered if the patches are itchy and transient, possibly related to an allergic reaction or physical stimulus like heat.

Stevens-Johnson Syndrome (SJS) / Toxic Epidermal Necrolysis (TEN): Although extremely rare and usually associated with medication use or infections, these conditions can present with skin lesions after various triggers, including possibly heat. They are medical emergencies due to their potential for severe morbidity and mortality.
Erythema Multiforme: This is an immune-mediated disorder that can be triggered by various factors, including infections and, less commonly, physical agents. It's crucial to distinguish it from other conditions due to its potential for recurrence and association with underlying diseases.

Cholinergic Urticaria: A type of physical urticaria that can be triggered by a rise in body temperature, which might occur during a warm shower. It presents with small, itchy wheals.
Aquagenic Urticaria: An extremely rare condition where water, regardless of temperature, triggers urticarial lesions. While the description doesn't perfectly match the question, it's a rare condition related to water exposure that could be considered in a broad differential diagnosis.