I normally feel asleep and weak when I'm in the mood to evacuate. Why? What could be the problem?

Dehydration. If you often feel sleepy or dizzy/faint right before moving your bowels that could be a sign of dehydration. When one bears down you often stimulate the vagal nerves in your neck which can cause a paradoxical decrease in blood pressure and heart rate. Dehydration often worsens this other important causes are medicines like blood pressure and heart meds. Check with your doctor though.