For over a month i have been feeling a stretch pain in my left back of my heel. i have flat feet, but have not had this issue before and for this long. when i walk too much i can feel the pain in the top of my left foot and even in my calve. ?

Functional flat foot: You are describing the bio mechanics of a very flat foot. It is not unusual for symptoms to present themselves at your age considering the way you describe your foot. I would try a semi rigid over the counter support to both feet to see if that helps your symptoms. stretches of the Achilles tendon is always appropriate