Why would one feel very tired, drained, drowsy before having a bm? normal stool but slow to pass.

This is unusual: and best evaluated by a doctor who knows you well. A few thoughts which I hope may be a little helpful: such systemic symptoms such as fatigue and drowsiness might suggest something chemical like carcinoid syndrome, but we would expect diarrhea/flushing, too. Inflammation may also be playing a role. I might also consider food allergy or intolerance such as milk/lactose intolerance. Work with doc.

This is a stretch...: but, when you strain to pass stool, you temporarily increase pressure on your heart. With a PFO, blood may be shunted from the left to the right ventricle or vise versa. Then, either oxygen poor blood or an insufficient volume of blood may be delivered to your brain & other vital organs which could cause your symptoms. if you also notice it at other times, please tell your cardiologist!