You may be reacting to soap or shampoo or something else other than or in addition to your bedding. Could also be dust mites that have gotten into your pillows or blankets (run pillows & blankets for 20 minutes in your dryer on a high/hot setting every 2 weeks to kill the nasty buggers).
Could also be delayed reaction to food. Could also be liver disease (one symptom of liver disease is itchy skin).
Have you seen a dermatologist or allergist who does skin patch testing (with food and non-food items)?
Do you have carpeting in your bedroom that isn't elsewhere in your house?
What else do you do before going to bed - handsoap, toothpaste, facial cleansing products and/or makeup remover, lotion, etc., that could be potentially causing or contributing?
Do you sleep with any pets in your bedroom - could it be fleas/flea bites?
And also you should ask your allergist for help in narrowing down the cause. Your doctor may also be able to prescribe medication to reduce your itching (such as long-acting antihistamines or antihistamine lotion/ointment).
I hope this helps - take care,
Gluten & Sulfite Sensitivity, Many Food/Inhalant/Medication Allergies, Asthma, Gut issues (dysmotility, non-specific inflammation), UCTD (Lupus?), Osteoporosis, Anemia, Very Low Lymphocyte Counts (T-Cells & B-Cells), malabsorption/malnutrition, possible Lyme Disease (Igenex Lab IgM WesternBlot positive), etc.
Meds: Pulmicort, MaxAir, Injectable Vitamin B12, Herbs, Nutritional Supplements, Essential Oils, Homeopathy.