Lupus Flare: Given the patient's history of Systemic Lupus Erythematosus (SLE), a flare of the disease could explain the soft tissue edema and pain, especially without a history of trauma. SLE can cause inflammation in various parts of the body, including the joints and soft tissues.

Gout or Pseudogout: These conditions can cause sudden onset of joint pain and swelling, and while they more commonly affect the big toe, they can affect other joints, including the ankle. The pain with plantarflexion could be indicative of inflammation within the joint.
Tendinitis: Inflammation of the tendons around the ankle could cause pain, especially with specific movements like plantarflexion. This could be related to overuse or could be a manifestation of the patient's SLE.
Ankle Sprain (missed trauma): Although the patient denies trauma, it's possible that a minor injury was forgotten or not recognized as significant. Ankle sprains can cause edema and pain, particularly with movements that stress the affected ligaments.

Deep Vein Thrombosis (DVT): This is a critical diagnosis to consider, as DVT can present with swelling and pain in the leg, and patients with SLE may have an increased risk of thrombotic events due to antiphospholipid syndrome. DVT requires prompt treatment to prevent pulmonary embolism.
Cellulitis or Soft Tissue Infection: Infection can cause edema, redness, and pain in the soft tissues. While less likely given the lack of systemic symptoms or a clear source of infection, it's a diagnosis that cannot be missed due to its potential for serious complications.
Osteonecrosis: Patients with SLE, especially those on long-term corticosteroid therapy, are at risk for osteonecrosis. This condition can cause pain and could potentially lead to edema if there is associated soft tissue inflammation.

Erythema Nodosum: A type of skin inflammation that can cause painful nodules, typically on the lower legs. It can be associated with various conditions, including SLE.
Sarcoidosis: A condition that can cause inflammation in various parts of the body, including the joints and soft tissues, leading to pain and swelling. It's less common but could be considered in a patient with SLE due to the overlap of autoimmune conditions.
Complex Regional Pain Syndrome (CRPS): A condition characterized by chronic pain, inflammation, and hypersensitivity after an injury or trauma, which the patient denies, but could potentially have been minor or forgotten.