7yr old may have cold back?

I've always understood "cold backed" to mean that the horse bucks under saddle consistently when first saddled or rode or hunches when saddled or rode. It doesn't sound like she's doing that and is happy to ride. I have a horse that has sensitive skin around his withers and flanks and will dip or arch his back and will twitch and shake like crazy for several minutes sometimes. When I asked the vet he said that some horses have more sensitive or thinner skin than others or perhaps even a greater amount of nerves around the back and spine which causes them to react to light touches (which is an unconscious motion to remove flies or other pests) more violently than other horses. The key to knowing the difference is to watch the ears and body language. If her ears aren't pinned and she seems relaxed she's alright, she's just mistaking you for a fly. :D LOL Try brushing with more pressure in that area instead of light touches. Don't be rough, just more firm. My horse twitches less that way.