Postictal State or Seizure: Given the patient's history of a right MCA infarct and new onset seizure-like activity, a seizure is a strong consideration. The generalized muscle twitching without focal neurologic signs could be indicative of a postictal state following a seizure. The seizure could be related to the previous stroke or other underlying conditions.

Syncope or Near-Syncope: The event occurred after stooling on the toilet, which could lead to a vasovagal response, especially in an elderly patient with atrial fibrillation. The brief loss of consciousness or near-loss of consciousness could be mistaken for seizure-like activity.
Cardiac Arrhythmia: Atrial fibrillation increases the risk of cardiac arrhythmias, which could lead to reduced cerebral perfusion, causing symptoms that might be mistaken for seizures.

Hypoglycemia: Although less likely, hypoglycemia can cause seizure-like activity, especially in elderly patients who may have altered mental status at baseline due to dementia. It's crucial to check blood glucose levels.
Intracranial Hemorrhage: Given the patient's history of atrial fibrillation and previous stroke, there's an increased risk of intracranial hemorrhage, which could present with sudden onset of seizure-like activity.
Infection (e.g., Urinary Tract Infection, Pneumonia): Infections can cause altered mental status and seizure-like activity in elderly patients, especially those with dementia.

Hashimoto's Encephalopathy: A rare condition associated with autoimmune thyroiditis, which can cause seizures, altered mental status, and stroke-like symptoms.
Creutzfeldt-Jakob Disease: A rare, degenerative, fatal brain disorder that can cause dementia, myoclonus (muscle twitching), and seizures, although it would be unusual to present with a single, prolonged seizure-like episode as the primary symptom.