Sharp pain on inside right heel?

Hi all, I have developed a sharp pain on the inside of my right heel. If you draw a line from the inside ankle bone to the bottom of the heel this pain is two thirds down towards the heel. I've never had PF or Achilles issues. No pain in the bottom of the foot. The pain feels like things are stretching and snapping and I only get it when extending the calf muscle with a bent knee. The pain is only present briefly after the ankle has been at rest for a long period e.g. getting up in the morning an dit goes within a second of two. No pain at all when running or stretching after. I'm happy to live with it unless doing so means I'm storing up an Achilles rupture in future weeks! Any ideas as to what it may be?