There can be a few different causes: for example, is it your shoes?, is it because of another health condition like Diabetes?, is it tendinitis?,...
I want to talk about the last one because I have personal experience with this one. If you have flat feet like me then the plantar fascia is constantly tight. When this happens it pulls on the attachments it has - you may get heel pain.
If you take an x-ray, you may even see a heel spur where the tendons and ligaments attach to the bone.