Thanks to all. Comments as appropriate.

Bed linens/different laundry detergent. I'd though of this. We use the same laundry detergent and cleaning process we always have. I've started laundering the bed linens more frequently since this started, and it's had no effect. My wife, who is on a low carb (Paleo) diet, has not had any rash or hives, and of course, she sleeps in the same bed.

Allergies to the food itself; Well, I thought of that, too. The only thing I'm eating more of is eggs. Not per day or per serving, but because I'm eating them every day, usually, for breakfast. That's different...I normally don't eat breakfast every day. So, my egg intake is up only because they've been eaten every day! But I haven't eaten breakfast (so no eggs) for the last two days, and got the hives again last night. What I didn't do yesterday was take a claritin-D.

High Histamine foods? I believe that this is part of the issue, and I've suspected it all along! I realize that meats are histamine sources (or is it that meat causes the body to produce histamines? I don't know the specifics...) But it's still puzzling, because overall I'm eating LESS meat than before. My calorie average for the last 10 days is 1271/day. That's with almost no carbs (less than 10g day). I'm not really starving, either...without the swings in blood sugar, I'm just not as hungry, and I'm eating smaller portions! So the meat intake is actually lower than before...but the proportions of meat to my total calorie intake is much higher! I suspect that the carbohydrates suppress this, when eating enough of them. I also suspect that over time, it would probably go away...so I'll probably continue with the Claritin D for a while longer, and then see what happens. I might also see what happens if I go up to say, 35 net carbs or so...at 1300 calories a day, I'll still be losing weight anyway.

Thanks to all of you that offered ideas. I see my doctor on the 20th. I'm going to run all this by him as well.