Why does the back of my heel hurts?

Find the cause... Heel pain can result from a number of different reasons. Common causes are bone spurs, inflammation where the tendon connects to the heel, bursitis and possibly even plantar fasciits. I would seek the attention of an orthopedic foot and ankle surgeon. One in your area can be found at www.Aofas.Org.
Plantar fasciitis. The most common cause of heel pain is plantar fasciitis: an inflammation of a thick band of tissue that helps hold up the arch of your foot. It attaches to your heel bone, and is usually the most painful for the first few steps in the morning, or after sitting a long time and then standing quickly. It may also be achilles tendinitis, or a combination of both.