Hi - not sure what it could be but because you can work it hard and not have pain during activity then you might try and figure out what specific movement or duration of movement set it off. Can you play one half of a game and not have pain after? Is it only rugby that brings on the pain later vs just running? If so then is it the deep ankle bend and forceful push off that might be the irritant during rugby? If so then you may want to focus on a particular exercise that improves tolerance for that particular movement. Do your own investigation and see what the culprit might be. Just a thought. -Best of health