Its a tricky one as it sounds unpredictable it could be one of a number of things. Could be peroneus problems a muscle that controls the movement of the foot in eversion and pointing your toes away and also stablising your foot. Its not uncommon to strain if you've been doing a lot of contouring or going from road type shoes to a fell shoe like a walsh.

Other thing it could be could be a nerve irritation as your peroneal nerve runs near there.

Its a bit tricky to tell really without looking at it. Carry on with other stretches and it might be worth playing around with calf stretching as well. Sorry I can't be much more help. sounds an interesting one to me.