The tibialis posterior is a little tendon imbedded around the ankle area that helps support the arch. It supports that rolling motion as you walk or run. My pobre tendon is stressed to the max trying to support my flat feet and sometimes it just gives up and goes on strike. I spent a good chunk of 2014 in physical therapy to address the issue. I remember my PT saying it would be something I would deal with again in the future but I was unprepared for how quickly it hit me again.
No warning. Just wham.One day I'm fine and am running 11 miles and the next I"m peg leg Sue, hobbling up to my bus stop.