What can cause sudden bad nausea hot/cold flushes and general malaise before and after a bm? no warning and can last 3hours after. i do have constipation problems but take lactose daily to keep under control?

If those symptoms occur after you take lactulose, side effects of lactulose should be ruled out. You can try a different laxative and see if you have the same symptoms or not. If you have not used Miralax, you can give it a try.

With nausea and "hot/cold flushes" after a bowel movement, best is to be referred to a GI physician for a complete workup.