I am not sure I'd be convinced it was the marijuana. It could be that you have gotten most of the stones out.
I use medical marijuana and used some the night of my first flush and will be using it during my next flush as well - it really helped me go to sleep (I have very bad insomnia). I have posted before that I wasn't sure my flush worked as I didn't really see stones but I think part of that was because I have been using supplements that may have helped "dissolve" the stones. Also I will be a little more thorough in my prep next time.