Anna Marie stayed at mom's house with Amanda tonight while Jason and I had dinner with my parents and my aunt (we took my aunt to get her cats from her brother, who had been "catsitting" them).
When we got back, little AM was in the tub, and I got her out to see the kittens. (KITTENS! FIVE OF THEM! OH, THE CUTENESS!) Within minutes of being dried off, she was hiving again.
I think it may be the detergent. It was Arm & Hammer, but I looked on the ingredients list and it had "perfumes and dyes" (nothing more specific than that.) The clothes she wore yesterday, as well as the towel she was dried off with tonight, were washed with that detergent. I don't think mom had used it before, and it isn't what we use at home.
Jason, bless his heart, came all the way over to mom's to bring her Benadryl. I left her there since it was so late, and mom promised to call me if anything changed. It's been an hour, and no call, so I guess everything is OK.
Maybe we've found the culprit after all.