The itching could be caused by Eczema. My son has it. He was a 34 week delivery and his skin has always been sensitive. It could also an internal food allergy. If it's worse at night, it could be dust mites.
It takes more than one washing to rid clothes and sheets of old detergent, etc. And switching products too soon doesn't help either. It's like testing for an allergy - consistent tracking of what you use and eat.
Here's what I would do and have done:
1. Use the Aveeno Eczema Body Soap - yes, it's more expensive, however, it works well on my son.
2. Use Arm & Hammer Sensitive Skin laundry detergent or Dreft. I have found that Tide, Cheer and All are all too harsh on his skin. I use it for everyone's clothes so there isn't more work for me.
3. Downy Fabric softner without Febreeze, scents, etc. keeps his clothes "softer".
4. Take the sheets off the bed, mix salt and baking soda together - shake it over the bed and "rub" it in - then vacuum the bed. This should rid her bed of dust mites (when vacuuming, ensure you have a new bag and have cleaned the brush of the vacuum).
5. Wash her pillows in HOT water. If she has sleepy toys (stuffed animals) wash them as well and limit to how many she can have on her bed and sleep with.
6. Find out how much diary she is having - my son can't have a lot of dairy in his diet. Not exactly sure what it is about dairy, but it helps to limit his dairy.
7. Ensure her room is dust free. You'd be surprised how fast dust mites multiply! I went to Target and bought the mattress and pillow cover for dust mites and allergens. I wash his bedspread every two weeks and change his sheets weekly.
If there's anything I can do to help you, please let me know. I live in Reston and know what it's like to have a young one "itchy" and being pregnant!