What Causes Tenderness Behind Leg After Prolonged Sitting?

2 months ago I started a job where I am sitting at a desk for a larger portion of my day than I had for the past 18 years. I noticed a definite difference within the first week. I have extreme tenderness behind my right leg right below my knee, to the extent that I need to stretch before I can put weight on my leg. I am also unable to kneel on my knees without increased pain. The pain is increasing worse and now goes up the back of my leg and down through my calf.