If your ankles hurt when running forefoot, certain parts of your forefoot strike landing pattern may be incorrect. For example, landing big-toe-first influences movement patterns of the ankle in ways that ratchet up ankle rolling, and the full consequence of this is an ankle sprain. To ensure stronger protections on the ankles during running, making initial ground-contact under the 5th toe was found to be a safer landing strategy of the foot. Read more here about the clear advantages and mechanical relevance of touching-down first under the 5th toe, not the big toe in forefoot running. 
Why Your Ankles Hurt When Running Forefoot
