It can be due to plantarfascitis which occurs due to inflammation in the cushion like structure present in the heel and pain aggravates when you start walking after a long rest. Or it can be due to tightness in the muscles of the calf.
Next Steps
Go to a physiotherapist.. Ultrasonic therapy +IFT +wax therapy +some stretching exercises will be helpful for sure.
Health Tips
At home.. Hot water fomentation +pain killer gel +frequent ankle movements before you start walking after a long rest and at every 2hours interval+wear soft footwears +use heel cushions+avoid walking on hard surface +no long standing.