I Really Need Help! It's Important?

I have been having problems with my horse, Raymey. She is a TB/QH cross, appendix QH if you prefer, she stands 16.1 hands tall or so and is jet black. She was once a polo horse. She is eight or nine years old, not registered. Anyway, lately she has gotten into the habit of refusing to work at random points in our rides. Sometimes at the walk, sometimes at the trot, and most often when I ask her to canter. She'll canter fine on one lead, but the other she refuses to pick up. She actually won't even canter on that track. But my biggest problem is when she does this at a walk or trot, because my beginner friend is taking her on a trail ride with me this weekend. I want to know why she's doing this. What she does is pin back her ears toss her head and refuse to go. I can MAKE her, but once had her rear when I tried. I don't want this to happen to my friend. I think she is trying to tell me something, but I'm having a hard time figuring out what it is. Her legs and hooves seem fine, though there is an old injury on one hind leg that is proud flesh now. Could that be bothering her? I've seen no other signs of lameness. She does, when she canters in the field, lower her head to a few inches above the ground and toss it. Does that mean anything or is she just enjoying herself? Another thing I've noticed is that she didn't used to do this. She would stop and not want to start again, but she was much less firm about not starting again. She has NEVER reared before that one time. Which is why I think something is wrong. I've checked her tack and it all seems to fit well, but I did get a new pad that is built up in the shoulders (I didn't know this on the phone with the person selling it and both my mom and my grandpa kind of pushed me to get it even though I wasn't thrilled about the built up front) and I was wondering if that could be making the saddle fit less comfortably for her. I've checked for injuries and sore areas and have so far found none. So, what do you guys think? PLEASE answer, I will choose best and answer any of your questions that I can!