Are you under any extra stress? This can sometimes cause the itching all over.
Back track to what you've ingested lately: new med? other symptoms?
If food sensitivities are an issue but you're not sure what the delayed reaction was: Do some research on the elimination diet. You may do better, too, if you see a naturopath for more thorough guidance through the process.
What kind of laundry detergent do you use? Try changing the detergent. Also, add an extra rinse to the cycle.
When drying your clothes, consider not using any dryer sheets. If you feel you need a fabric softener, use white vinegar in place of liquid fabric softener. Your clothes will not smell like vinegar, I promise. I actually use no form of fabric softener most of the time. The only time I use a fabric softener is the white vinegar in the washer. Truly, it is not really necessary to use fabric softeners. All fabric softeners do is add chemicals to your clothes.
So, keep looking for various chemical agents in the home that you are exposed to on a regular basis. You will have to keep looking. Change over as many things as you can think of to more natural ingredients, because this will help remove what is bothering you, if it's an external source. Even an unopened container of something you rarely use will leach out chemicals into your living environment, so keep this in mind as well.
You don't have to have a rash to have an allergy.
See my post above to GeorgeSS