My new horse was given her strangles shot in the neck two days ago and today I rode her and her back end seemed very sore and her canter seemed to hurt her. She wouldn't stay in it. So I got off her and lunged her around and it seemed like her back end was stiff and sore. She also wouldn't flex her neck very well. Could this be from the strangles shot? She has never taken a sore step in the 6 months I have had her.