It sounds like you may have torn your Achilles tendon. The Achilles tendon is the tendon that connects the calf muscle (gastrocnemius) to the heel bone (calcaneus). This is the tendon along the back of the ankle.

If the tendon has not ruptured, then you may have strained the tendon. This type of injury results in a stretch injury to the tendon called tendonitis. In some cases, the tendon itself doesn't tear. Instead it pulls a piece of calcaneal bone off of the rest of the calcaneus.

You'll need to see a medical doctor for a proper diagnosis. The tendon may heal on its own without treatment. For some conditions, early treatment is the key to success.

In the case of Achilles tendon rupture, a cast may be needed to immobilize the foot during healing. Sometimes physical therapy is advised. When conservative treatment doesn't help, surgery is a final option.