Plantar fascitiis if it is on the bottom , if in the back an achilles tendon or haglunds deformity.
A stress fracture
of the heel or less likely but possible a bone tumor
If you get pain to the heel on the first few steps after resting likely plantar fascitiis. Icing of the heel, limiting walking while pain is severe, a old arch support, an antiinflammatory
are all good ideas.