Is she perhaps getting sour?
It sounds as if she's learning to hate being ridden, which is so wrong---horses should learn to enjoy it. Mares especially, I think, sometimes need to be met through the "back door"--that is, not using punishment, so much as somehow finding something satisfying for them to do, then rewarding it.
Even if you can back-'er-and smack-er through it, her cooperation would be less than ideal.
Assuming she's not hurting in any way, I suggest doing something different--anything--that she is willing to do, and edging back into her normal routines.
One more note: it's not to say you shouldn't carry a crop--you should, and be able to use it too. But I feel that when you do, you must accept what she does, no matter what--there has to be a short moment of freedom for her, before you take up where you left off.