[Foolkiller]

I can only guess this happened due to having your instep under the knee a few times rather than your shin. It's probably just your achilles tendon playing up. Something to remember is always make sure your toes are pointing upwards so you're not effectively achilles locking your own foot by overhooking it with your knee. Keep your toes pointed and your foot pointing upwards. This will always make your calf tense up making the pressure against the back of his neck stronger and ensure your instep is not going to take the strain.