A thought...drink lots of water before and after your Epsom Salts . The salts causes water in the body to go to the bowels, which is why you have to go so badly (fact). But if you don't have enough water in the system, you can get badly dehydrated if you don't also drink water as well--which is why you can feel so lousy; the body is struggling to retain it's vital water reserves while the Epsom Salts are busy yanking the water out...(my belief based on how I felt not drinking/drinking water with the es).
This is my opinion, not medical fact, but I will tell you on flushes where I drank extra water I felt MUCH better with the es.