What Causes Severe Pain At The Posterior Of Calcaneum While On Treadmill?

last october i ran 6km on sand for first time. [usually i run on a treadmill] with no problem. then later in the day felt pain at posterior of calcaneum. walking makes it worse, but again pain comes on later. the pain is a stinging kind of pain and can be felt on the skin. The pain takes weeks or months to fade, but is very easy to aggravate by even a small amount of walking. I am age 49. 11st3. what is causing this pain, many thanks. ultrasound scan didnot seem to show any problem with achilles tendon, but may show slight problem with retrocalcaneal bursa between tendon and calcaneum. if i squese both sides of tendon there is no pain. it feels like it is just under the serfice of the skin