The tendon on the outside of your knee is the MCL - Medial Collateral Ligament. 3 things that jump to mind can cause soreness in it:
1. Saddle too high.
2. Overtraining - too many miles, not enough rest.
3. Cleat positioning on your shoe - a lot of people don't have feet aligned naturally straight forward, so if you mount your cleats in a way that forces your leg into an unnatural position you'll put stress on the joints. Solution: free floating pedal system such as Speedplay, fitting at a (professional) bike shop, manual adjustment of cleat (but experience helps a lot here).