This condition is called plantar fasciitis. It's not uncommon in flat foot. Also seen in people who have normal arches in foot. Pain is severe when you wake up from bed and take few steps and then gradually decreases. Also seen when you sit for sometime and start walking. Physiotherapy like plantar fascia and calf muscle stretching exercises, alternate hot and cold fomentation, soft cushioned heel and analgesics for few days will help you.