My thoroughbred mare has an awful habit of throwing her head towards me like she's going to nip while I'm brushing her neck and sides, almost as if to say, "I don't want you to touch me cause it hurts." and I've had vets check her out and they say she's fine. She only just started this about 2 years ago and I've had her since 2001! She's been with only me since then so I don't know why she would all the sudden start doing this, I haven't changed the way I do things so it doesn't make any sense. But besides that I've tried everything, from growling at her, to saying, "NO!", to slapping her mouth and to no avail. Any help on this situation would be much appreciated.