I second the recommendation that you request a referral to a good physical therapist.
You might also check your library for some of the Leslie Sansone Walk at Home videos. They are very easy on the knees and provide as much or little of a workout as you want. I especially like the 2011 5-Really Big Miles (it's broken up into 1-mile segments) for a number of reasons, including the diversity of steps and a slightly dancier feel without complex choreography.
Good luck figuring out the problem with your leg. Don't do anything that hurts!!