Hypothyroidism exacerbation: Given her history of hypothyroidism, a common cause of fatigue, and the fact that hypothyroidism can fluctuate, requiring adjustments in medication, this is a highly plausible explanation for her symptoms. Lightheadedness could be related to decreased cardiac output or other metabolic effects of untreated or undertreated hypothyroidism.

Anemia: Despite a normal colonoscopy, which reduces the likelihood of a significant gastrointestinal bleed, anemia from other sources (e.g., menstrual bleeding is ruled out due to hysterectomy, but other sources like urinary tract or skin lesions could be considered) or nutritional deficiencies could cause fatigue and lightheadedness.
Vitamin Deficiencies (B12, Iron, etc.): These are common causes of fatigue and can be related to dietary deficiencies, malabsorption, or other factors. Given her age and the fact that she's had a hysterectomy, ensuring adequate nutrition and investigating for deficiencies is reasonable.
Sleep Disorders: Conditions like sleep apnea or insomnia can cause significant fatigue and should be considered, especially in a post-menopausal woman where sleep patterns can change.

Cardiovascular Disease: Although less likely given the information, cardiovascular diseases (e.g., heart failure, coronary artery disease) can present with fatigue and lightheadedness. Missing these diagnoses could be fatal, so they must be considered, especially in a post-menopausal woman where the risk of cardiovascular disease increases.
Adrenal Insufficiency: This is a life-threatening condition that can cause fatigue, lightheadedness, and other symptoms. Given her history of hypothyroidism, which can be associated with other endocrine disorders, this should not be overlooked.
Cancer: Although her colonoscopy was normal, other types of cancer (e.g., lymphoma, multiple myeloma) could cause fatigue and should be considered, especially if other symptoms or signs are present.

Pituitary Disorders: Conditions affecting the pituitary gland, such as a pituitary tumor, could lead to hypothyroidism (if affecting thyroid-stimulating hormone secretion) and other hormonal imbalances causing fatigue and lightheadedness.
Chronic Fatigue Syndrome: A diagnosis of exclusion, characterized by profound fatigue that is not improved by rest and lasts for more than 6 months, among other criteria. This should be considered if other diagnoses are ruled out.
Mitochondrial Myopathies: Rare genetic disorders affecting the mitochondria, which can cause fatigue among a myriad of other symptoms. These would be considered if there's a family history or other specific findings suggestive of a mitochondrial disorder.