What Causes Fatigue And Light Headedness After Bowel Movement?

Hi, I feel real fatigue, sleepy and light headed after poop (bowel movement) in the morning. After poop rest of my day is spoiled. My poop is normal not so liquid neither too hard though whatever the shape or color is I have this problem. Also it is in normal quantity neither too much nor too little. I feel this way sometimes everyday of the week and sometimes just once or twice a month. I do not smoke nor am i alchoholic and do excercise thrice a week.
Thank you in advance.