Does sound like it could be shivers. Here is some info about it -
"Shivers or shivering are names that have been applied to a chronic nervous or neuromuscular syndrome in horses. The syndrome principally affects the draft horse breeds. Shivers has been also reported in Warmbloods and Warmblood crossbreds, and occasionally in lighter breeds of horses, including light harness horses, hunters, hunter-jumpers, hacks, Quarter Horses, and Thoroughbreds. In ponies, shivers is considered uncommon to rare. The condition develops slowly and can occur at any age with reports in horses as young as one to two years of age. Many horses that hold a hind limb flexed and tremble are said to have Shivers. However, many disorders can create irritation in the hind limbs of horses and cause this type of sign. The classic disease called Shivers is a neuromuscular disease in horses that is characterized by trembling of the tail while held erect, trembling of the thigh muscles and a flexed and trembling hind limb. (UNIVERSITY OF MINNESOTA Equine Center)"
Hope that helps.