The commonest cause of heel pain, especially when stretching is Plantar Fasciitis. The plantar fascia is a strong membrane that stretches from the heel to the base of the toes. It can get inflamed if too much strain is put on it as by wearing very high heels, obesity, a job that requires long hours of standing, arthritis, heel spurs etc. Stretching the leg out causes the plantar fascia to get stretched and ppain.
You need to get this checked up by an orthopedist (bone doctor). A physical examination, and tests like Xrays and MRIs will help in the diagnosis.
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I hope this helps.
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