Is it possible the hives were already, under her hair? Perhaps it's a reaction to one of the vaccines, since they were given during the last 90 days.
Or, perhaps it's a reaction to the fungal shampoo. Why did she need to be bathed in it?
I'll keep racking my brain..worked with an Arab who did this about this time every year (in Texas), but I can't remember what pasture plant Harlan said it was. Not nettles. We did ease it by rinsing with 5 gallons of water that had 2 cups of apple cider vinegar added. Sponge it over her until it starts to bead off--you'll be able to tell the difference--and if you don't use it all, save for next day.