The problem with using sheets for a backing is the thread count. Flannel is a different animal. Cheap cotton sheets that have a low thread count will work for a quilt backing that will be handquilted. Flannel is a much looser weave. It will shrink so be sure to prewash it several times and dry on the highest setting.
Your hands will become accustomed to being stuck. I have callouses on my finger tips on both hands as I have trained myself to quilt with both hands. Just becareful if you bleed on the quilt. Your spit will remove your blood. I know that sounds odd but it really works.