Help! My horse is so stiff on the right side, she's uncomfortable lunging and it causes her to pace!?

I've owned my horse for 6 weeks now, and we've gotten over her being riding sour, and are working on w/t. But for the past week it seems she's taken 8 steps back. Before, pacing instead of trotting was unusual, now its the only gait she'll pick up. So she was trotting and now she's pacing. Also, when I lunge her she's pretty good on the left side, and I can get her to trot. But on the right you can tell she's so uncomfortable. She hollows her outline and can only shuffle along at a pace, and then she'll randomly turn away from the lunge as if to avoid bending. This is only on the right side, and picking up the trot under saddle is much harder than on the left. She's not a standardbred, so I'm wondering if this pacing fixation is because she's in pain from something or really stiff for some reason. IDEAS?!