Hi, I Have Severe Stiffening And Pain In The Back

Hi,
I have severe stiffening and pain in the back of my ankles above the heel. This occurs after sitting in the car, sitting in general and on waking. The first few steps are painful with tightening from the heel up my calf. Once I have walked for a bit the pain and stiffness goes. I have not had any injuries. What could be causing this? It t came on fairly suddenly and has worsened over time.