Cellulitis: This is a common bacterial skin infection that causes redness, swelling, and warmth of the affected area. It is often accompanied by fever and malaise. The most likely diagnosis due to its high prevalence and the symptoms described.

Deep Vein Thrombosis (DVT): A blood clot in the deep veins of the lower extremity can cause swelling, redness, and warmth. Risk factors include recent surgery, immobilization, and cancer.
Superficial Thrombophlebitis: Inflammation of a superficial vein, which can cause redness, swelling, and warmth along the path of the vein.
Eczema/Dermatitis: Skin conditions that can cause redness and swelling, especially if there is an allergic component or infection.
Insect Bite or Sting: Certain insect bites or stings can cause localized redness and swelling.

Necrotizing Fasciitis: A severe bacterial infection that destroys tissue under the skin, requiring immediate medical attention. Symptoms include severe pain, swelling, and redness.
Gas Gangrene: A potentially deadly condition caused by Clostridium perfringens infection, characterized by severe pain, swelling, and crepitus (bubbling sensation under the skin).
Septic Arthritis: A bacterial infection of the joint, which can cause redness, swelling, and severe pain. Prompt diagnosis and treatment are crucial to prevent joint destruction.

Erythema Nodosum: An inflammatory condition characterized by red nodules under the skin, often on the lower extremities, associated with various systemic diseases or infections.
Lymphangitis: Inflammation of the lymphatic vessels, which can cause red streaks and swelling, usually as a result of a bacterial infection.
Kawasaki Disease: A rare condition in children that causes inflammation in the walls of medium-sized arteries throughout the body, leading to symptoms including redness and swelling of the extremities.