A few things:
Is your DS sick? Running a fever or acting "off"?
Is your DS formula/BF/solid eating?
What detergent are you using?
I'll check back later on you.
Ann-
It could be that your DS is just peeing on the stains already in the diapes. They could be really faint after washing when diapes are "clean" and then show up when they're wet. IMO. Sounds like you've R/O anything health related, so this must be it.