Deep Vein Thrombosis (DVT): Given the patient's history of cancer, which increases the risk of thrombosis, and the presence of unilateral leg edema, DVT is a highly plausible diagnosis. The symptoms of dizziness and nausea could be related to a pulmonary embolism if the DVT dislodges, or they could be unrelated but coincidental.

Chronic Venous Insufficiency: This condition could explain the leg edema, especially if it's chronic and related to the patient's history of hypertension and possibly immobility following her CVA or cancer treatment.
Heart Failure: With a history of hypertension and CVA, heart failure is a possible diagnosis, especially if the patient has developed symptoms of fluid overload like leg edema. Dizziness and nausea could be symptoms of decreased cardiac output.
Postural Orthostatic Tachycardia Syndrome (POTS): Although less common, POTS could explain the dizziness, especially if it worsens with standing. However, it would not directly explain the unilateral leg edema.

Pulmonary Embolism (PE): While this might seem like an extension of the DVT diagnosis, it's crucial to consider PE separately due to its high mortality rate if missed. Symptoms can be non-specific, including dizziness and nausea.
Stroke or Transient Ischemic Attack (TIA): Given the patient's history of CVA, another vascular event is possible. Dizziness and weakness could be indicative of a new stroke or TIA, especially if the weakness is focal.
Intracranial Hypertension: This condition could cause dizziness, nausea, and even papilledema. It's less likely but critical not to miss due to its potential for severe complications, including vision loss.

Superior Vena Cava Syndrome: Although rare, this condition could cause edema, particularly if the patient has a history of cancer that could lead to obstruction of the superior vena cava. However, symptoms would typically include facial and arm edema rather than isolated leg edema.
Lymphedema: This is a possibility, especially if the patient has had lymph node dissections as part of her breast cancer treatment. However, lymphedema typically develops over time and might not fully explain the acute onset of other symptoms like dizziness and nausea.
Nutritional Deficiencies: Certain deficiencies, like vitamin B12 deficiency, can cause neurological symptoms including dizziness and weakness. While less directly related to the edema, it's an important consideration in a patient with a complex medical history.