I would start with Naturopath horse chiropracter. They are living beings and she might have taken an impact to the shoulder-scapula putting out thorasic 1 or withers. Their is a better health detox product for horses at Home Page on horses page. email me a small jpg pic of side shot of horse standing with head up in a natural stance to april@treelesssaddle.com and I will let you know if I see anything relevant.
do not give up
check out my utube stretches video's-free at Holistic Horse Works as well