Fractured my ankle back in July. Healed fine but yesterday after a high impact workout, I am having bad pain & hurts to walk on it. Why is it hurting?

Take it easy. While bones themselves typically heal by 8 weeks, the associated tissue damage (and pain) can potentially last longer. The muscles are also likely weak from disuse. Ease back into HIIT slowly - give your muscles and soft tissue time to regain strength.