Epsom Salts are used to dilate the bile ducts but also have the added benefit of quickly 'clearing the way' through your colon. What you describe is pretty much normal.
It's a *good* thing to get the colon as empty as possible before drinking the OO/GF mixture as it allows the released 'stones' and other old gunk a fast track out. Having those released toxins hanging around in your system can really make you feel miserable and lousy.
It may help to eat lightly the day before the flush, then to eat sparingly or even fast the day of the flush to lessen the 'explosive' effects of the Epsom Salts .