Pancreatitis is the most likely explanation for left side/center pain and it's a common complication of Gallstones and I think we are respondling to your "I feel like I am dying!!" comment and assuming there is something fairly severe going on. If you are just having pain and that gets better, then whatever caused it moved or resolved and you will probably be OK.
Center could also be gallbladder too, maybe your pancreas was irritated after the flush and now it's just your gallbladder -- hard to say & none of us are doctors here. The feeling like something is stuck is probably gallstone/gallbladder related if it's up high under your ribcage. When my gb was blocked, I felt like I had a baseball sitting up under my ribcage, center, but just the right side.
Just be careful that you are not constipated when you flush or there is no where for the stones and toxic matter to go & it just gets reabsorbed.