It could be a number of things. Perhaps you might consider what may have changed in your life when it started to hurt.
It could be as Rugelyboy has said, or, for example, it could be plantar fasciitis (PF) as pointed out above. Repetitive activity can cause the pad of our heels to stretch. Sometimes when this happens, small tears appear in the pad which becomes inflamed. This usually happens near or at the point where the pad is attached to the heel, which is why it feels painful. The symptoms can also be caused by the frequent wearing of high heels, ill-fitting trainers or through age and weight. PF is an inflammation of the pad, whereas heel spur is to do with the heel bone itself.
Arnica cream and witch hazel solution are good natural anti-inflammatory treatments. Paracetamol and ibuprofen work well, too. Other treatments include putting a foam ring support in the heel of the shoe to keep the pressure off the painful area (homemade or bought from a pharmacist), acupuncture, physiotherapy or even steroid injections can be used (usually only once other treatments have failed).
The best person to advise would be your GP.