I have a 16 year old fox trotter/walker mix. I rode her in a parade and we did nothing but walk and at the end, she came up lame on her rear left leg. Nobody knows what happened to her, and 2 weeks later she's still limping a little. There's no swelling or visible marks, and she still wants to go go go but after she does, she still acts like it hurts. What should I do?