Hard to tell you how to treat it via a forum without an assessment. If there is full range in the keen when there is no weight on it then the tightness could have been coming from your low back. That tightness could then have upset the normal mechanics of the knee movement and resulted in some patellofemoral pain due to a little overuse in an incorrect motion. Think friction irritation.

Maybe an initial approach would be to do low back extension exercises to see if you can resolve the tightness while resting the knee a little to see how much natural healing you can get. A few days off this week will only help your Mudder event. If you can just book into your local sports PT just for an assessment and guidance on who to approach it for this week as they might be able to pinpoint the specific issue for you :-)