This has just seem to happen to after my 4th flush. I did some parasite cleaning this time to make sure I would get max amount of stone. I think this might be the reason why. But you said you never did any parasite cleaning. One thing that works great for me is presciption Denvir. If you put it on when you feel the tingle it never comes for me. And when I dont get it in tiime and it forms. It only takes like 4 to 5 days to heal. Much faster than normal for me.