An allergist should be able to determine the cause.
The only way to do this yourself is to keep a diary.
Buy an unscented detergent to start. I would not suggest Tide, as many people seem to react to it.
Try something natural. Use this on all your bedding and clothes for a week. If it stops the itching, you have found your cause. If it continues, try another detergent.
Keep at this until you find one that works.
Of course, it's not 100% that it's an allergy to detergents/perfumes/soap/shampoo etc. It could be the food you eat.
So you could try eating nothing but rice for 3 days. Add another food at that point. Eat only those 2 items for 3 days, then add another , and so on. Food allergies can be instant or take up to 72 hours to kick in...hence the adding something new every 3 days.
In other words? You'll either need to see an allergist or spend the rest of your life ( possibly ) trying to figure it out.
Sorry I can't be of much further help.
The Alternative therapies forum might be able to help you more.