I had purchased California Baby shampoo, which is supposed to be hypoallergenic and mild and such. I've used it the last three times I've washed my hair. I think I'm going to trade off between that and the baking soda. So hair washing problem: solved.
This leaves me with a larger, more difficult problem - what AM I allergic to?
Hypothesis 1: Laundry detergent.
Supporting Evidence: We use a granular detergent and I know that it doesn't always rinse completely out because I've seen granules at the end of wash cycles. We have a teeny, tiny washer so it's possible that I've been using way too much detergent and putting in too many clothes to prevent full rinsing.
Experiment 1: Change laundry detergents and start double-rinsing everything. Also, add vinegar to the rinse to help remove old detergent build-up.
Results so far: Inconclusive. Some things after double-rinsing didn't seem to make me itch, while other things still did. It may take more than one laundering to fully remove any build-up of old detergent, so this experiment is still in progress. My three-or-four-times-laundered PJs don't seem to make me itch much.
Experiment 2: Do a home allergy test by applying a paste of old detergent to the skin of my inner arm.
Results: Small amount of redness and itching apparent. There was definitely more than the new detergent or vinegar, my control.
Hypothesis 2: Food.
Supporting Evidence: None, except that the rash doesn't seem to appear in the areas I would expect if it was an allergy to laundry detergent. I would imagine that the rash would be worst in areas that are closely contacted by clothes (waistband, underwear, socks), but that doesn't necessarily seem to be the case. I've also never had an allergy to any other foods before.
Experiment: Eliminate all the usual suspects as far as food allergens. This includes dairy, wheat, soy, caffeine, chocolate, alcohol, and basically any other food that's fun to eat. Here's a complete list if you're interested.
Results: I started the diet on Thursday, so it's been 5 days so far. Basically this is how we ate over the summer: lots of veggies, with rice and quinoa as the major grains. I haven't had any major cravings, although I would like to eat some things that aren't on my list (yogurt is the biggest one). No major changes noted.
Hypothesis 3: The degu.
Supporting Evidence: I'm allergic to rats and mice; degu are also rodents.
Experiment: Home allergy test done by rubbing a degu on the skin. I vigorously applied Vu to my stomach, including some small scratches with her claws.
Results: There was a small amount of redness and itchiness, especially around the scratches. However, it's hard to say whether this was due to the degu-ness of the application or the scratches themselves.
Being an excellent scientist, I'm doing all three of these experiments at once so that I can change as many variables as possible. I'm working on the theory that I can take everything out and get rid of the allergies (the goal), then add things back in one at a time until I figure it out. It was way more important to get rid of the itchiness than to do a well-designed experiment.
Complicating interpretation of the results is the fact that I've started to take allergy meds. I wanted to sort of rest and reset my immune system with the idea that maybe it will stop over-reacting to whatever the allergen is once stop taking them. I'm going to take them for a week, while continuing with the hypoallergenic diet and change in laundry detergent / technique. After a week, I'll stop and see if the itchies come back.