She may have pinched nerve, cracked ribs, ulcers, or who knows what.
It could also be you are irritating in your mannerisms, and voice is pitched too high, or you could be too "busy" in your body movements.
Also, this mare is very old and may have been hurting for a long time now, and this is only way she can get point across.
As for your comment that mare is definitely not in pain?
How do you know that? Has an equine vet seen her, and done diagnostic testing to determine that? X-rays? MRI's? Blood testing?
If those have not been done? Then you have no idea how much pain the mare may or may not be in.
Furthermore, I can assure you that the older you get, the more pain you do feel. Joints are stiff, and achy, old bones hurt more with the cold, and every step can be agony too. Which is what I imagine this mare is going through.
This horse is at an age that I doubt she wants to be ridden much at all, if any. Hitting her is not going to change that.
Horses make me a better person.