There are a number of injuries that can cause pain in the buttock and upper leg. They are typically caused by overtraining or by running on slippery surfaces. They are described at this page. That page also gives treatment information, including photographs of stretches, but you will have to register for a free membership to access the treatment section. One of the stretches is the Gluteal, and I do it before and after each run. Here is another web site that gives photographs of buttock stretches.
The information on this site is for informational purposes only; it does not constitute medical or physical therapy advice. For medical advice, consult a physician. For physical therapy advice, consult a physical therapist.