This may sound like an odd question, but have you ever had your water tested? In our neighborhood, iron in the water is a problem. Some of the wells have manganese as well. When iron in noticable amounts is exposed to sunlight, it will become even more noticable. I never gave it much thought until I moved here, but one of the neighbors and I became friends and she had a horrible problem with this. It ruined nearly all of her sheets, blankets and towels. When she noticed the spots on her clothes she started washing her clothes anywhere she could to keep them from being ruined. There is a product called iron out that I have used and it helped with the spots greatly.

If you want to see if you have iron in your water without the expense of a test, you can fill a clear jar with tap water and leave it sit in sunlight for a couple of days. The sooner it turns a yellow/orange color the higher the iron content.

I don't know what else would cause the spotting you are talking about, but this is my limited experience. Hope it helps.

Did you use bleach? I washed a pillow in a fairly strong solution of bleach-maybe one cup in a full washer.(I was hoping to sanitize it.) Instead the pillowcase turned nearly orange. I had to throw it away. No one wants neon linens! On the other hand, I bought some flannel sheets from Lands End. After several washings, they pILled very badly. I wrote to Lands End. They sent me a refund plus a free second set! Do you know who manufactired your sheets? Just a couple of suggestions.

Iron? An interesting question. I have city water but live in an old building with rusty pipes. It's a possibility, but I have not noticed nearly as much of a problem with underwear/undershirts and socks. However, I do get faint rusty stains in the toilet, so may be? Would the yellowing and staining show up after time?

And yes, I probably did use quite a bit of bleach during the last sheet laundering, so I was wondering if that could be it, since bleach has a reputation for yellowing. The strange thing is the delayed reaction -- Does bleached laundry look o.k. when it comes out of the wash and then darken with time?

The set of sheets on the bed were not matched. The percale fitted sheet and pillowcase looked so awful that I threw them out -- The flannel oversheet should be salvageable, maybe with Oxygen bleach? All the sheets were 100% cotton.

Depends on what the sheets are made of. Is there a tag on the sheets advising not to use bleach? Some fabrics will turn yellow if bleach is used on them.

People who smoke usually will have stained sheets, more of a yellow brown color.

A build up of detergent can do strange things too.

Try soaking and rinsing well in vinegar water several times to get out detergent residue.

Sorry it has taken me so long to get back to you. Yes, iron can happen that way and it is sometimes less noticeable on things like underwear and socks because they are not exposed to sunlight. I have seen that bleach is also a problem as mentioned here, but it does seem to show up when the clothes come out of the dryer. I can also agree with the detergent build up. Try washing anything that is yellowing with half the amount of detergent and an oxygen bleach with a cup of vinegar in the wash and skip the fabric softner. Wash it in the hottest water you can get in your machine. If you have a top loader, maybe a pot of hot water will help. I use vinegar as a fabric softner and periodically will use regular bleach as opposed to the oxygen bleach for some clothes and even with the iron in our water, I seem to have less problems with the yellowing of clothing than most of the neighbors.

Sometimes trial and error will eventually tell you what caused it before you have it happen again, but hopefully you can figure it out without having any more clothing ruined.