Are you using any benzoyl peroxide on your face/body?
Or any hair products that may be bleaching your sheets?
Are you on any medication that could be seeping out of your pores and altering your body chemistry.

It could be anything.

I've seen white underwear that's washed using bleach also bleach the color out of dress shirts that are tucked into pants and residual bleach lightened the waist area of the shirt.

I only use white sheets as they're easier to keep clean and white.

Sounds like bleaching caused by benzoyl peroxide (common ingredient in acne medications/lotions). Usually it will not be visible until you actually wash the affected item. Do you have dark color towels and are you seeing the same issue on them?

Could be whitening toothpaste -- if you drool in your sleep, the residual toothpaste can bleach the sheets. Took me a while to figure that one out!