How long have you had her? When did this attitude change start? What was she eating at her old barn? What is she eating now? (hay, grain, supplements, amounts of all of these) How much turnout does she get? How much did she get at her last barn? Has she gained or lost weight recently? How does your saddle fit RIGHT NOW? Have you had it checked by a saddle fit recently? (not your trainer/instructor, a real saddle fitter)
Before adding a supplement, you need to figure out exactly what's causing her behavior. A small change in feed, turnout, or saddle can make all the difference in the world.