I never felt anything whatsoever on my right side untill my first Master Cleanse which I only lasted 5 days on too.
The way I understood it was once you start flushing, there's an empty spot left where the stones are and as it fills up again or stones roll back down, you feel the ache. I don't know though. Makes sense if you started with flushing but I started having it with fasting. Then I did a flush a week apart from another because I had that ache.. But didn't even really pass stones, passed mucus.