This common condition, if left untreated will plague you for years and rarely disappears on its own. Many people come for treatment saying they HAVE had it for years.
Classically the pain occurs first thing in the morning and sometimes after a short rest. The pain tends to go after a while but returns on resting again.
The most likely cause is a tear or rupture of some of the fibres of the long ligaments that reaches from the front of the foot to the heel that maintains the arch (known as the plantar fascia, the painful condition being called plantar fasciitis).
Management of the foot function is essential to start helping this condition, this could be with special insoles and/or strapping. Some people also respond well to stretching exercises but these must be done in a controlled fashion to avoid further damage.