What causes the feeling of weakness and nausa during and after a bowel movement?

Vagus nerve issues: Having a bowel movement activates the vagus nerve and overstimulation of that nerve can cause weakness and nausea. See http://www.brainprotips.com/vagus-nerve-symptoms/ and http://drsircus.com/general/function-vagus-nerve/ for details. Conventional medicine does not really address this, but acupuncture, homeopathy and a treatment called neural therapy may help address this.