It is most likely due to plantar fasciitis. This problem is a result of excess stress through a long ligament type structure (the plantar fascia) in the bottom of the foot. The excess stress causes tearing and results in inflammation and pain. The classic symptoms are pain in the heel at the first step in the morning, or upon rising after long periods of rest.
Most common reasons for the development of plantar fasciitis is wearing poor quality or worn out shoes. Another common reason is starting a new activity such as walking or running, after a period of inactivity.