changes? changes in saddle, riding time, ?
how long did you saddle her that she did NOT react this way?
is she starting any other behaviors that are pushing on you a bit.?
sometimes a horse is bought who has great manners, but the new owner does not know how to enforce them, so little by little, the horse is allowed to get away with more and more, until new owner notices some bad habits.
but, if the hrose is flinching when touched (firmly), then consider physical reasons for this response and consider tack changes, perhaps not tacking up correctly (saddle not positioned correctly or girth too tight or?)