Do you get your water from a city supply or is it well water?
Well water is famous for discoloring things.
My mother-in-law used to get a rusty tinge to her laundry from well water.
If it`s not your water supply,
how often are you washing your whites?
You need to wash them every time you wear them.
Don`t throw them in your hamper.
Wash them the minute you take them off.
If you don`t have the chance to throw them in the machine then hand wash.
Your body oils and the salts in your deo discolor white fabric if left to sit on it without washing.