Hello , Post bypass surgery many reasons can be the reason for drowsiness . Most common and easily correctable are low sodium or other metabolic reasons ( hypoxia, low sugars , impaired renal and hepatic causes) As you said pain medication or sleeping pills , some antibiotics and other rarer medication which can cause the same. Considering her history of stroke and epilepsy, A EEG to rule out silent continuous seizures . Lastly during bypass the blood supply to the brain is drastically decreased , do an MRI to rule out any new stroke , or another hemorrhage considering the blood thinners used for bypass patients should be the later considerations if other common causes are ruled out. Regards