Why does my dog limp sometimes?

Is it a hind leg? If it's occasionally limping on a hind leg, the likely explanation is that your dog has a luxating patella, or a kneecap that slides in and out of place. If they have issues with the ligaments in their knee, or the anatomy of their joint is defective (whether from birth or due to injury) it can cause this to happen. They'll be walking fine one minute, limp for a few steps until they can pop it back into place, and then they act as if nothing happened. It's definitely important to consult a vet about this, as it can progress and cause more serious issues, such as complete lameness. The earlier you address an orthopedic problem, the better!

Limping is caused by a variety of problems, such as injury, infection, inflammation, anatomic defect, cancer, and degenerative diseases. ... In dogs, arthritis (sometimes referred as osteoarthritis) and injury are two of the most common causes of lameness.