Why is my mare suddenly acting like she's mental?

Today I was riding my Thoroughbred mare like I do 4 days a week and sometimes more. When I asked her to canter and asked for left lead she picked up her right. Which is weird because shes never gotten her leads wrong before. So I slow her to a trot and ask again and she does it again. This went on for thirty minutes with my trying all kinds of different tactics but she absolutely refused to pick it up. And she wasn't angry, or bucking, or acting abnormal at all, she just wouldn't get the left lead at all. Shes healthy as far as I know, the only thing I can think of is that she got into some brambles or something and her left hind leg has some scratches on it but she hasn't be limping or acting like it was bothering her. And she was perfectly fine picking up her other lead, she wasn't reluctant to canter. Why do you think shes acting like this all of the sudden and what can I do about it?