I have a new feather pillow, it’s covering fabric was white when purchased. I put a clean pillow case on the pillow, used it for awhile, then took the pillow case off to wash it--after removing the pillow case I noticed the pillow fabric has small black/brown/rust colored spots all over it, on both sides, yet the pillow case is free from any sort of spots-so it could not be something spilled on the pillow. What caused the spots?