Something similiar happened to a mare of mine last year. She looked like from her throat to her tail and down to her knees/hocks, she had misquitoe bites everywhere! I fought with this for 3 weeks, had the vet out, used cortisone cream on her entire body, used benadryl in her feed, everything I could think of. Finally I started to remove supplements from her feed so all she was eating was grain, then I added them back in slowly from the order of what I had been using the longest on her. When I got to the last thing (which was black oil sunflower seeds) she broke out again! This was my problem and this is how I found out she was highly allergic to them. Not saying it could be just about any kind of an allergic reaction, but maybe she is eating something new, either a supplement or maybe something she found outside or something someone else might be giving to her...
I also thought the antihistamine would take care of mine, but soon found out that if the allergen is still getting to the horse, then this is not true!
You know how to make a miniature horse even smaller? Leave them in the dryer a little longer!
"Don't ever regret something that once made you smile"