Just because I'm curious, where did you get your flush recipe? (It seems I've read it before, but can't place where right now.)
I've never really experienced any pain at all, aside from a slight burning around my 'bottom' after releasing a lot of toxic stuff into the toilet. I have had some nausea, but that was due to my own experimentation in taking too much oil or sneaking a snack after I was supposed to stop eating.
Like Sarape noted, one key step is to eat NO fats or oils the day of the flush. The only fat or oil should be the olive oil at the end of the evening. This allows a build up of bile during the day to help 'push' the 'stones' out.
I've always basically done The Clark Flush that uses half the amount of olive oil which is mixed with fresh squeezed grapefruit juice and doesn't include coke at all. (Personally, after reading the ingredients in them, I haven't had a carbonated softdrink in nearly 2 years so I completely avoid including one in a liver flush. Just seems counter-intuitive to me to add additional chemicals to an already over-taxed and 'stone' filled liver.)
Also (just to nit-pick) the oil doesn't actually go to the liver or gallbladder. (Over-simplifying here) The liver produces bile which normally 'drips' down into the gallbladder until needed, but increases under a higher fat/oil load. When fat or oil enters your stomach a 'signal' is sent that tells your gallbladder to empty the stored bile and your liver to produce more bile to restock the supply. (This is why a flush still works to cleanse the liver even if the gallbladder has been removed.) 1/2 cup of oil is usually a large enough amount to signal the gallbladder to dump all it's 'held' bile as quickly as possible. It's this build up and sudden release of bile that 'flushes' the 'stones' out.
In many flushes, another key ingredient is magnesium (whether Epsom Salts or magnesium citrate) to relax the no-striated (smooth) tissues (like the bile ducts) thus allowing them to relax enough to allow larger stones to more easily pass through when being 'pushed' by the sudden relase of built up bile. This is also the reason drinking a Tablespoon of Epsom Salts in a cup of water or apple juice helps relieve a gallbladder attack.
Epsom Salts also have the additional affect of 'clearing a path' though the intestinal system so it's less likely the released 'stones' and their accompanying toxins will linger around in your system making you feel ill. (Epsom salts are commonly used to help relieve constipation - and boy, do they! lol)
However, like I said before, I'm curious and always open to learning new ways to do things, especially when it comes to health. I'm very interested to know how your flush goes!