On cotton, a type of oxidation can occur called "foxing". It may have been caused by acid-treated paper in contact with with the cloth during the time when you had it put away. There are also some types of mildew that can form due to moisture inside a plastic wrapper, and this can have a bleaching effect, making your fabric turn from blue to yellow.
Either way, you're probably looking at color damage, and possibly fiber damage; not a "stain" that can be removed.