Based upon a bit of rudimentary first aid experience, I'd suggest that the bruising might indicate a minor tear to a ligament, probably at the point that it attaches to the bone. (Based upon Doctor Google, that might well be your anterior talofibular ligament).
It can be amazingly easy to damage a ligament. I once had to deal with a boy who'd done nothing more than fall over while playing football, yet his knee ligament had completely detached itself from the bone. Similarly, my aunt had to give up riding her moped because simply twisting the throttle damaged the ligament in her wrist.
What sayeth the mighty Sqad to my suggestion though, I wonder?