My Dog Occasionally Trips On Her Front Legs When She

My dog occasionally trips on her front legs when she walks. Twice now I have recently noticed her staggering and seeming disoriented. Our veterinarian has done x-rays, a physical exam, even put her on the medicine for Lyme disease for a week even though she tested negative. Bloodwork in April was good. Vet says next step is a CAT scan, but could we be overlooking something simple. She is eleven years old and a husky/German Shepherd mix.