Could it be plantar fasciitis?

Plantar fasciitis refers to the syndrome of inflammation of the band of tissue that runs from the heel along the arch of the foot; a heel spur is a hook of bone that can form on the heel bone (calcaneus).

Plantar fasciitis is most often seen in middle-aged men and women, but can be found in all age groups. Plantar fasciitis is diagnosed with the classic symptoms of pain well localized over the heel area of the bottom of the foot. Often the pain from plantar fasciitis is most severe when you first stand on your feet in the morning. Pain often subsides quite quickly, but then returns after prolonged standing or walking.

Plantar fasciitis is sometimes, but not always, associated with a rapid gain of weight eg during pregnancy.