I tend to agree with alophysio. Sounds like a loose body or even a subluxation of a tendon like the one of the peroneals. it is common to have a small additional piece of bone on the calcaneous (heal bone) which can fracture off. This is called an os trigonum and is a common issue with ballet dancers. It's removal is quick and simple with a rapid recovery. I agree you should have an investigation perhaps with the foot pointed (even if you can bring on the symptoms and then scan or x-ray). Should that show nothing then this may just be a chronic instability that could benefit from some surgical intervention to prevent the excessive movement. Do let us know how you get on.