Hypovolemia: Given the recent surgery, the most likely cause of hypotension in this patient is hypovolemia due to blood loss or dehydration. The patient's age and the fact that she had surgery the day before increase the risk of hypovolemia.

Bleeding: Post-operative bleeding is a common complication of surgery, especially in elderly patients who may be on anticoagulants or have other coagulopathies.
Anesthesia-related complications: Residual effects from anesthesia, such as vasodilation or decreased cardiac output, could contribute to hypotension.
Pain and anxiety: Underlying pain or anxiety from the surgery could lead to hypotension, although this is less likely given the patient's current vital signs.
Medication side effects: Certain medications, such as anesthetics, analgesics, or antihypertensives, can cause hypotension as a side effect.

Sepsis: Although the patient is afebrile, sepsis can present atypically in elderly patients, and hypotension can be an early sign. It is crucial to consider sepsis, especially if there are any signs of infection or if the patient's condition deteriorates.
Pulmonary embolism: Given the patient's recent surgery and immobility, pulmonary embolism is a critical diagnosis not to miss, as it can present with hypotension and tachycardia.
Cardiac complications: Myocardial infarction or cardiac tamponade can cause hypotension and must be considered, especially in an elderly patient with possible underlying cardiac disease.

Adrenal insufficiency: Although rare, adrenal insufficiency can cause hypotension and should be considered if the patient has a history of steroid use or adrenal disease.
Neurogenic shock: This is a rare cause of hypotension but could be considered if there was any spinal or epidural anesthesia used during surgery that might have caused sympathetic blockade.