Wow! Sounds like you had a great flush! Congratulations!
Liver Flushing can be draining for the kidneys, sounds like the lower back pain is related to Kidney problems.
Most people are recommended to do kidney cleansing BEFORE doing liver flushing, and you need to do it after every 3 or 4 flushes also.
Simple things like eating lots of parsley, taking Watermelon seed tea, eating watermelons with seeds, drinking Celery seed tea, drinking cranberry juice and also drinking LOTS of good water will help the kidneys. Also, stay off meat and dairy as kidneys can't cleanse with rich animal protein.
Maybe you didn't break up the stones enough. Did you use stone breakers like gold coins grass, chanca piedra, lecithin? Maybe the ACV in The Master Cleanse wasn't enough - not sure. Maybe you just have LOADS of stones!
I've nearly done 20 flushes and each flush is different, sometimes large sometmes small and sometimes none at all.
I find an ice/cold pack is MORE effective for pain than hot pads. I asked my chiropractor about this and she also said that ice packs help relieve pain and hot packs don't, so try that.