While riding my 17 yr old half-Arab, half-Saddlebred mare yesterday, she stumbled and then started limping. I dismounted, felt her legs and shoulders. While unsaddling her, she stood with her weight balanced nicely on all four legs. Unable to feel any heat or swelling, I gave her a gram of bute and put her back in the pasture. Today when I brought her up from the pasture, she seemed fine. I trotted her out and she appeared sound. I saddled her up and took her on a short trail ride, trotted occasionally to test her soundness. She did fine. She even galloped down a short stretch of trail. When we arrived back at the stable, I took her into the indoor arena for a few moments to prevent barn sourness. She stumbled (different spot) and began limping again, both ways, though more pronounced to the right. Again THERE IS NO HEAT OR SWELLING OR ANY PAIN DETECTED!! What could this be?