Your fitted sheet is worn out but the flat and pillowcases still new! It's very common that fitted sheet wears out much sooner than a flat sheet and that is due to that fact that fitted sheet is subjected to body and feet friction much greater than a flat sheet. This Luxurious Hybrid Bamboo-Cotton Fitted Sheet gives you the option to just replace only what you need. Made from 60% Bamboo Viscose, 40% Cotton 

and woven at 300 Thread count per square inch puts this in a class about the rest.