Based on the history you provided, your kitten may have sustained a bone or muscle bruise, but falls or traumas can also result in more serious injuries, such as ligament tears or fractures.
Below is a link for a common injury in the rear limbs of cats who have recently sustained trauma for your information.:
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