Cellulitis: This is a common bacterial skin infection that can cause redness, swelling, and pain in the affected limb. The absence of injury does not rule out cellulitis, as it can occur through minor, unnoticed breaks in the skin or via hematogenous spread.

Deep Vein Thrombosis (DVT): Although DVT typically presents with swelling and pain, redness can also be a feature, especially if there is associated inflammation. The absence of injury does not exclude DVT, as it can be precipitated by factors such as prolonged immobilization, cancer, or genetic predispositions.
Phlebitis: Inflammation of the veins can cause pain, redness, and swelling. It might not always be associated with an injury and can be related to infections, blood clots, or inflammatory conditions.
Lymphangitis: An infection of the lymphatic vessels, which can cause redness, swelling, and pain in the affected limb. It often originates from a skin infection and can spread along the lymphatic pathways.

Necrotizing Fasciitis: A severe infection that destroys tissue under the skin, including fat and the tissue covering the muscles (fascia). It requires immediate medical attention and can be fatal if not treated promptly. The presence of severe pain out of proportion to the physical findings is a red flag.
Septic Arthritis or Osteomyelitis: Infections within the joint or bone, respectively, which can cause pain, swelling, and redness. These conditions are medical emergencies, especially if not responding to initial treatments or if the patient has a compromised immune system.
Compartment Syndrome: A condition resulting from increased pressure within a muscle compartment, which can lead to muscle and nerve damage. It is characterized by the "5 Ps": pain, pallor, pulselessness, paresthesia, and paralysis. Although it often follows an injury, it can also occur without a clear traumatic event.

Erythema Nodosum: An inflammatory condition characterized by the sudden formation of painful, red lumps on the skin, often on the shins. It can be associated with various conditions, including infections, medications, and systemic diseases.
Thromboangiitis Obliterans (Buerger's Disease): A rare disease of the arteries and veins in the arms and legs, which can cause pain, swelling, and redness due to inflammation and clots in the blood vessels. It is strongly associated with smoking.
Complex Regional Pain Syndrome (CRPS): A chronic pain condition characterized by severe, burning pain at the site of an injury, which can also include swelling and changes in skin temperature and color. The cause is not fully understood, and it can sometimes occur without a clear precipitating injury.