What Causes Severe Knee Pain When Lifting Leg Or Walking Up And Down Stairs?

I am a commercial roofer and last Friday I was pushing wheelbarrows full of river rock. Sometimes this included running with them due to time. On Saturday I started to have pain in my right knee. It is on the outside of my knee and I get a severe sharp pain while walking up or down stairs, and when lifting my leg and bending it backwards. What could this be?