Post-traumatic ankle swelling: Given the history of an ankle/Achilles injury 5 weeks ago, the most straightforward explanation for the diffuse edema is a post-traumatic response. The lack of pain but presence of swelling could indicate a chronic inflammatory phase following the injury.

Deep Vein Thrombosis (DVT): Although less common in the context of an ankle injury, DVT should be considered, especially if there's been prolonged immobilization or other risk factors. The edema could be a sign of a DVT, which might not always be painful.
Cellulitis: An infection of the skin and subcutaneous tissues could present with diffuse edema, redness, and warmth. The absence of pain doesn't rule out cellulitis, as the condition can vary in symptom severity.
Chronic Achilles Tendinopathy: This condition could lead to swelling around the ankle and shin area due to ongoing inflammation of the Achilles tendon. It might not be painful at rest but could cause discomfort with activity.

Compartment Syndrome: Although typically very painful, compartment syndrome is a medical emergency that could potentially present with swelling and less pain if the patient has a high pain tolerance or if it's in a very early stage. It's crucial to assess for this condition due to its potential for serious complications.
Osteomyelitis: Bone infection could be a complication of the initial injury, especially if there was an open wound. It might present with swelling and could be relatively painless in its early stages.
Popliteal Artery Aneurysm or Arteriovenous Fistula: These vascular abnormalities could potentially cause edema due to altered blood flow. They are less common but critical to diagnose due to their potential for severe complications.

Eosinophilic Fasciitis: A rare condition characterized by inflammation of the fascia, which could present with diffuse swelling and might follow trauma. It's an unlikely cause but should be considered if other diagnoses are ruled out.
Lymphedema: If the injury affected lymphatic vessels, it could potentially lead to lymphedema, characterized by chronic swelling. This would be an unusual complication of an ankle injury but could be considered in the differential diagnosis.
Sarcoidosis: A systemic disease that could cause swelling and might be triggered by trauma. It's a rare condition and less likely to be directly related to the ankle injury, but it could be part of a broader differential diagnosis if other systemic symptoms are present.