What Causes Dizziness Before Passing Stool ?

I m having dizziness before I go to the bathroom #2. I feel faint weak hot flushes and cold sweats before I defecate. This has never happened before. 2 years ago I had half my colon removed because of sepsis a week after having my gallbladder removed. I have a family history of colon cancer, father late 40 s. Have always had problems with bowels but this is different, almost feels like sepsis. My bowel habits have changed since having hemicolectomy just in the last few months, more constipation and pain. Any idea? Thanks very much!