What Causes Pain In Knee Radiating To Calf?

My left knee was very sore yesterday after I got home from work, a drive of about 40 mins. It feels quite tender when I bend it or kneel on it and I have a sort of dull light throbbing sensation travelling down the calf area of my leg. I can walk on it but bending it is painful, never had this problem before. I am 63 years old and female.