Anemia or Iron Deficiency: This is a common issue post-surgery, especially after a significant procedure like a double mastectomy, due to blood loss during or after surgery. Fatigue is a hallmark symptom of anemia.

Hormonal Changes: If the patient had hormone receptor-positive breast cancer, the sudden drop in estrogen levels after surgery (especially if they were premenopausal) could lead to fatigue, among other symptoms.
Depression or Anxiety: The psychological impact of a cancer diagnosis and subsequent mastectomy can be profound, leading to mental health issues that manifest as fatigue.
Sleep Disturbances: Discomfort, pain, or anxiety related to the surgery can disrupt sleep patterns, leading to fatigue.
Medication Side Effects: Certain medications used in the treatment or prevention of breast cancer, such as tamoxifen or aromatase inhibitors, can cause fatigue as a side effect.

Recurrence or Metastasis of Cancer: Although less likely, new onset fatigue could be a symptom of cancer recurrence or metastasis, especially if other symptoms like weight loss or pain are present.
Infection or Abscess: Post-surgical infections can occur weeks to months after surgery and may present with non-specific symptoms like fatigue, especially if the infection is deep or chronic.
Deep Vein Thrombosis (DVT) or Pulmonary Embolism (PE): These are potential complications in post-surgical patients, particularly those with limited mobility, and can present with fatigue among other symptoms.

Adrenal Insufficiency: Though rare, this could occur if the patient received steroid treatment preoperatively and then had it abruptly discontinued, or in cases of metastasis to the adrenal glands.
Hypothyroidism: Either as a result of treatment (e.g., radiation therapy affecting the thyroid) or as an unrelated condition, hypothyroidism can cause fatigue.
Nutritional Deficiencies: Deficiencies in vitamins like B12 or folate can lead to fatigue and are more common in patients with poor dietary intake or malabsorption issues post-surgery.