What Causes Pain And Stiffness In Knee?

I have been training for the marathon and did some hill work on the weekend which haven t done before. I think the force put on my leg has caused an issue. Ran 5 days later and did 10 miles with a slight niggle. When I stopped, leg was instantly very painful and couldn t straighten. Wore off but still sore when I extend around knee. It hurts if I tap the outside of my knee next to the kneecap. Any ideas? Have iced it and still a dull ache.