What Causes Pain In The Buttocks Region While Stretching Or Running?

In the interior of the buttocks region close to bone I have a pain that gets worse with stretching or running. It does not hurt standing, walking or sitting but feels more like a tugging or pulling of tendon or muscles. There is no pain in back or leg. But if I stretch the leg in certain ways, I feel it in the sore region of the buttocks. I have had it for about 5-6 days and tried running today but pain became more intense so I stopped.