Lymphedema: Given the patient's age, lack of other medical history, and negative tests for common causes of edema like DVT and heart failure, lymphedema is a plausible diagnosis. It could be primary (inherited) or secondary to another condition, though the latter is less likely given the absence of other medical history.

Cellulitis or Infection: The slightly elevated CK and low platelets could indicate an infectious or inflammatory process. Cellulitis could cause leg edema and does not require a history of trauma or surgery.
Nephrotic Syndrome: Although renal function is reported as normal, early stages of nephrotic syndrome might not significantly alter renal function tests but could lead to edema. The slightly low platelets could be seen in some cases due to the loss of proteins that help regulate clotting.
Sickle Cell Disease: Given the patient's Nigerian origin, sickle cell disease is a consideration, especially with slightly low platelets, which can be seen in this condition due to splenic sequestration. However, this would typically present with more systemic symptoms.

Deep Vein Thrombosis (DVT) with Negative US: Although the ultrasound was negative, DVT cannot be completely ruled out, especially if the patient has risk factors not mentioned (e.g., recent travel, family history). A negative D-dimer or further imaging might be necessary to fully exclude this diagnosis.
Ovarian Torsion or Other Gynecologic Emergencies: These conditions can cause leg edema due to compression or obstruction of lymphatic and venous return and are emergencies that require immediate attention.
Malignancy: Though rare in this age group, malignancies (e.g., lymphoma, sarcomas) can cause edema due to obstruction of lymphatic or venous structures.

Fabry Disease: A genetic disorder that can lead to edema among other symptoms, due to the accumulation of a particular type of fat in the body's cells.
Klippel-Trenaunay Syndrome: A rare congenital malformation that affects blood and lymph vessels, leading to edema and other symptoms.
Lipedema: A chronic condition characterized by the abnormal growth of fat cells in the legs, leading to swelling and edema, often affecting women.