Most commonly, pain in the heel has a gradual onset without any history of trauma to the area. The pain is progressive over a few months and is typically felt under the inner aspect of the heel border. Usually, it starts in one heel only and is worse upon weight-bearing first thing in the morning. It generally eases with activity but becomes pronounced at the end of the day or after a long day on your feet.
It is caused by an overpull of the tendon that attaches into the calcaneum (heel bone), this is called the plantar fascia and therefore inflammation of the tendon is called plantar fasciitis. It can also be accompanied by cramping of the calf muscles as tightness of the Achilles tendon is a major contributory factor.
The underlying position of your foot is often to blame for this condition and therefore advice on shoes and insoles is important. If your foot pronates (i.e. that flattens excessively on standing) this may need to be addressed by using orthotics.
Seeking the advice of a chiropodist / podiatrist early gives the best chance of resolving your heel pain completely.