Bloody hell! Ice and rest the hell out of it if you can. It is the insertion for yoyur achilles, and you need to ice and rest it as its a very avascular structure, and damage takes a long time to heal. Meanwhile put a flat insert in your shoe under the heel, called a heel raise, about 5-7mm thick should ease it a little as it takes the stretch out of the tendon. Do this for both feet otherwise you could stress the tendond/ligaments/muscles of the opposing knee and hip. If still bad after a few day get and see a podiatrist.
Ibuprofen and paracetamol if its painful.
A Podiatrist.