You might have a deformity known as a haglund's deformity or "pump bump: - it a protrusin of bone to the back of the heel.
If this is it, you would want to try and wear backless shoes until the pain goes away, you can ice it as well. If it is a constant pain that doesn;t seem to ever go away you may want to contemplate surgical excision. (assuming this is what you have.).