It could be that the saddle no longer fits her and is causing pain. Make certain that there are no "lumps" in the saddle and have a saddle fitter look at how it fits your horse. Also - make certain that you are not placing the saddle too far forward - the part of the saddle near her withers MUST be behind the top of her shoulder.
My saddle had to be adjusted 3 times in one year as my mare was developing so much muscling in her topline the saddler had begun to pinch her and cause her pain. She also reacted as soon as she saw the saddle pad - snapping at me, etc. I fixed the saddle and after a few times she realized it no longer hurt - thus no more snapping.
If she react to grooming it may be too much/not enough selenium or she could be ovulating and in pain. Be careful about selenium - VERY easy to OD the horse on it. Try the Chastberry herb (1 tsp 2x/day) if it's an ovulation related issue.