Brief Answer:
Plantar fascitis may be the cause

Detailed Answer:
Hello,
Thanks for writing.
I have gone through your query carefully and the problem you are having is medically known as plantar fascitis.I hope you are medically fit (non diabetic) and have normal weight. In this earlier you start the treatment better the prognosis.The other less common cause could be infection,for this you may send the picture of heel,If possible.If it is a infection it would be warm and very tender to touch along with redness,do you have that?
My advice for you would be-1.Rest your feet. Limit or, if possible, stop daily activities that are causing your heel pain. Try to avoid running or walking on hard surfaces, such as concrete. Never walk bare foot that is put on your shoes as soon as you get out of bed.
2.Wear shoes with good shock absorption and the right arch support for your foot. Athletic shoes or shoes with a well-cushioned sole are usually good choices.
3.You may use silicon heel cups to cushion your heel. Use them in both shoes, even if only one foot hurts.
4.The usual medicines you may have are Ibuprofen/naproxen to relieve pain along with tablet tendocare under medical guidance.
5.You may do exercises like toe stretches, calf stretches and towel stretches, several times a day especially in the morning. For towel stretches, you pull on both ends of a rolled towel placed under the balls of foot.
6.Avoid using only heat on your foot, such as from a heating pad or a heat pack for at least the first 2 or 3 days. Use hot and cold fomentation (alternate hot and cold water, make sure you end with a soak in cold water). 
Hope this helps and I shall be glad to answer any further query.
Regard

---