Is this lameness?

The last couple days my mare has seemed...lame? At a walk and trot she's actually fine. She doesn't resist weight on the foot, she doesn't drag it, or swing it out. But at a lope she seems to miss her step with it, and she stays low... I need to get a video, it's hard to explain. And it only when the 'problem' foot is on the inside of a circle. Has anyone ever seen this? I plan to give her a couple days off and if there is no change I'll call the vet....but I would like to know what we could be getting into.

Thanks