Cellulitis: This is the most likely diagnosis given the presentation of cellulitis, which is a common bacterial skin infection characterized by redness, swelling, warmth, and pain in the affected area. The typical causative organisms are Streptococcus and Staphylococcus species.

Eczema (Dermatitis): Can present with similar skin changes, including redness and swelling, but usually has a more chronic course and is associated with itching.
Contact Dermatitis: An allergic or irritant reaction to substances coming into contact with the skin, which can mimic cellulitis but typically has a clearer history of exposure to an offending agent.
Insect Bites or Stings: Can cause localized reactions that might be confused with cellulitis, especially if there are multiple bites or a severe reaction.
Superficial Thrombophlebitis: Inflammation of a vein just under the skin, which can cause redness, warmth, and swelling similar to cellulitis but is usually associated with a palpable cord.

Necrotizing Fasciitis: A severe infection of the fascia, the tissue that separates soft tissue and organs, which can present similarly to cellulitis but is much more severe and can lead to significant morbidity and mortality if not promptly treated.
Gas Gangrene (Clostridial Myonecrosis): A rapidly progressing infection of the muscles, characterized by gas production, which can start with symptoms similar to cellulitis but quickly becomes severe.
Deep Vein Thrombosis (DVT): While DVT typically presents with swelling and pain in the leg, it can sometimes be confused with cellulitis, especially if there is associated inflammation. Missing DVT can lead to pulmonary embolism, a potentially fatal condition.
Erysipelas: A type of skin infection that involves the upper dermis, typically caused by Streptococcus, and can be distinguished from cellulitis by its well-demarcated borders and usually affects the face or legs.

Erythema Migrans (Lyme Disease): The rash associated with early Lyme disease can sometimes be confused with cellulitis, though it typically has a distinctive "bull's-eye" appearance.
Sweet Syndrome (Acute Febrile Neutrophilic Dermatosis): A rare condition characterized by fever, neutrophilia, and tender, erythematous skin lesions, which can mimic cellulitis.
Pyoderma Gangrenosum: A rare skin condition that can cause painful ulcers, which might be confused with severe infections like cellulitis, especially if there is significant surrounding inflammation.