Pain, upper left butt cheek. Not a cyst. Hurts to move lower body. Heat/ice doesn't help. No stretch helps. Advil=temporary relief. Lasted 3 months?

Gluteal pain. Gluteal pain can be caused by various conditions. A few of the common ones: gluteus medius muscle strain (can cause pain in the upper outer buttock), referred pain from the lumbar spine (i.E a pinched nerve). Other causes can be piriformis syndrome, sacroiliac joint dysfunction, hip disorder, and others. Discuss with your md for a thorough history, exam, diagnosis and appropriate treatment plan.
Sacroiliitis. Sounds like either a facet syndrome or possibly sacro-iliitis. Both tend to be subacute to chronic pain producers. A pelvic x-ray is reasonable after 3 months of pain, and some physical therapy may be able to produce a remission, depending on what is found. Next step is a clinical examination with your doctor.