Based on the history you provided, your dog may have sustained a bone or muscle bruise. Such mild injuires can have exaggerated consequences due to underlying Arthritis that is common at this age. Please note that falls or traumas can also result in more serious injuries, such as ligament tears or fractures. With limping resulting from any significant trauma, a veterinary visit is advised. If you would like to consult with a veterinarian 24 hours a day, 7 days a week, please refer to our AskTheVet page