How did the yellow stains end up on your clothing? You don’t remember it being there before. More than likely they were caused by oxidation.
Here is a scenario: You are out to dinner and a bit of white wine spills on your shirt. You dab it with some sparkling water and hang it up to dry at the end of the evening. The next day the shirt looks great. Fast forward 6 months and you go to put the shirt back on and now it has yellow stains on it.