I was grooming Tara earlier and as a started brushing her gaskin on her hind right leg, her leg started quivering and she sucked it away from the brush. As I went farther down her leg she was back to normal, so it's just when I touch her gaskin. I can touch it very lightly and she doesn't react, but anything harder and her leg shakes and she pulls it away. No kicking though, she doesn't have a problem putting weight on it, and from what I can tell she's not lame, though I didn't trot her out. Also no cuts or scrapes or anything like that.
What do you think could cause this? I was thinking maybe she got kicked and it's just really sore?