Is she pacing?

I really doubt she's pacing, but she is probably modifying the canter to favor her injured leg, which is probably not as strong as it once was. I had a mare who damaged her extensor tendon in a hind leg, then every time she would canter on the lead where that leg was pushing off on it's own, she'd speed up the other hind leg so that it was coming down right after the weaker leg, helping it along.