Groundwork Issues

So, before doing our riding out in the neighborhood, my trainer has had me lungeing Licorice. She's typically pretty responsive on the ground, but for some reason has been giving me guff the past few days. I really wonder if she's sore, or just being a snot. She walks, jogs, and trots both ways nicely. The extended trot is kind of heavy on the fore, but for now, I'm just trying to work on responsiveness. Trouble comes when I ask for a canter... she spins toward me and gives me a snarly mare face, even going so far as to bite the lungeline! She goes to the right easier than to the left... she wasn't doing this before, so I wonder if it's a soreness issue. But.. yesterday I worked her through it... basically, I kept bringing her back in to me and sending her out every time she balked, and after about 10 repetitions of this nonsense, she cantered on the left lead. She was also trying to canter to the left in a right lead... then finally switched to the left.

Trainer thinks she is just testing me, and says she has seen horses go out on the wrong lead before because "they know they aren't supposed to," and it can be another way of testing the owner or trainer, and that they anticipate being stopped and asked again for the correct lead... basically, another way to get out of the work. Licorice is well known for trying multiple ways to get out of doing something before finally giving up and doing it. And the thing is, she looks fine, not lame or gimpy, once she finally complies, and in fact, can collect nicely on a loose rein.

Anybody have any ideas for how to really tell if it's pain versus just attitude? I live in a small area and I'm not sure how readily available and equine chiro would be just for a consultation.