Hoof problems.

I have an 11 year old appaloosa mare named Rosie. About two months ago she started limping, favoring her front left, but only when she trots. I thought perhaps it was a stone bruise or maybe she had been kicked. She continued to limp when trotting until one day she could hardly walk on it. We immediately went to the vet. There was no heat in her leg or hoof, and when he numbed the hoof, she trotted perfectly. He also X-rayed the hoof and found nothing wrong. He told me to wait a little longer, and give her the pain killer, but still she limps when she trots. There is no odor and her hooves look to be in good shape. I'm especially worried because her mother was put down due to hoof issues when she was only two months old. Does anyone have any ideas as to what could be ailing my mare?