I had a TB mare that out of the blue broke out in hives. I thought she somehow developed an allergic reaction to flies or bugs, but it turned out that I started using a different fly spray and her allergic reaction was to that. Have you started using a different fly spray, or using a different sheet or saddle blanket? Is she in a new pasture where she may be rubbing against different types of flora she’s not used to? I know these questions may be longshots, but I hope they helped. Good luck.