if you have flat feet, then that is DEF the problem, because after a while, your muscles will cramp up and just pull the fuck outta your leg/heel. i would reccomend you go to walmart and do the Dr. Scholls foot analizer thing. buy the insoles, and do these stretches when your boots are OFF before riding.
1. stand facing a wall/locker or something, about four or so inches away from it.
2. bend your foot so its up against the wall
3. lean forward for 20-30 seconds and repeat 5 or six times with each leg
4. this stretches your tendons in your back leg/heel region and with everything i mentioned, your problem should be fixed,
i had the same problem for a while and after doing this i had little to no pain whatsoever
7SpRiNgS K2 TuRbOdReAm