The reasons why the dog suddenly became lame without any trauma:

1. Soft tissue injury: For example, the dog accidentally hit its own muscles or ligaments when running and jumping, causing pain and leading to lame. This situation usually resolves in five to seven days.

2. Joint pain: This is common in some Poodles, Bichons, and small dogs. Especially for patellar luxation, if the dog suddenly lifts its feet after running for a while and does not dare to touch the ground, but suddenly recovers after a while, it is necessary to consider whether its patella is dislocated.

3. Femoral head: Many dogs will have congenital development problems or avascular necrosis in the femoral head, and they will also experience pain.

4. Arthritis: This is common in old dogs. For example, the joints of the limb bones or the joints of the spine cause pain, resulting in lameness of the hind limbs. It is best to take X-rays and then respond accordingly. Treatment.