I too was wondering if it could have been the backing fabric that was the culprit. Was it washed? Is it light/white on the backside? I was thinking it could have been the actual fabric weft that was being exposed by quilting it. Maybe the needle hole was causing the backside of the backing fabric to show through? I hope you can figure out why/how this happened and maybe when it's washed, it won't hardly be noticable. Such a shame that this happened to you.