Is my horse having a reaction to a sedative?

I wouldn't be overly worried since he may have pulled a muscle during the procedure, or one of the shots may have affected a nerve in his neck and be bothering him. Some horses react differently to the medication and don't excrete it as rapidly, but it wouldn't usually still be affecting him today. I think it is good to have a talk with the vet just to be sure, but if the horse is eating well I doubt it is anything to worry about. Just watch his water intake to be sure it resumes normally. Also, if any signs of colic develop, which is always something to watch for when a horse acts differently from his norm, and especially if he isn't drinking fluids, be sure to remove his feed and call the vet right away.