Achilles Tendonitis: Inflammation on the Achilles tendon, connecting the calf muscle to the heel bone, usually caused by overuse. This brings about pain and stiffness within the back of the heel, especially all through activity. It's prevalent to slumber with ft pointed down, which shortens the plantar fascia and achilles tendon. https://www.theorthopaedicandpainpractice.com/why-does-the-back-of-my-heel-hurt/