I had a lot of burning with my third flush, which seemed to have flushed out much older stones.
There were no stones,only chaff and foam with my first flush. Lots of yellow tan gravel with my second flush, but no difficulty, burning or nausea with either of the first two flushes.
My third flush was very different. I released some red smooth stones and small black stones in addition to the gravel and had intense buring of the anus as well as weakness, nausea and headaches on the day after the flush. I wasn't using a collander, so I could only identify the stones I could see. Apparently I released many that sank because I clogged the toilet with my watery bowel movement (had to get out the plunger).
I found some relief by applying coconut oil to the anal area before and after each BM until the burning let up. Coconut oil is cooling.
Also, I did a week of internal oleation prior to the third flush, which helped build up fresh bile to help push things through on the third flush. I believe this helped the third flush to cleanse more deeply, but may have contributed to the burning.