Oh, there is an injury, just not one you can see on radiographs or she wouldn't be limping. Have you had her checked out by a chiropractor?
It could be a muscle tear or stain. Something slightly out of aligement. They won't show up on radiographs and unless you can feel the swelling or they react darn hard to find. So could be mild inflammation in a joint. lots of thing that might not show up on tests
I know how frustrating it is. Justice went thought this limp, no limp thing for 6 months after his flexor tendon injury. He rehab for 4 month prior looked good for 3 months and than started on/off limping. No one could find out what the issue was. What finally did the trick was a trip to the chiroprator and 8 injections of adequan plus rest.