The Initial Symptoms Are Mild
Most people do not develop Achilles tendonitis at once. The condition develops slowly and progressively worsens. This is characteristic of most injuries to the tendon.
The symptoms of Achilles tendonitis usually begin as a mild ache at the back of your leg, just above your heel. Initially, you may only feel this pain shortly after running or playing aggressive sports. 
It’s hard to avoid using your heels every day so the condition worsens with time. As this happens, it may become even hard to climb the stairs or jog a small distance.
Many people don’t realize they have Achilles tendonitis when they first develop the condition. They may dismiss it as a muscle problem since the attendant pain and stiffness often subside after warming up.