image provided by Wikipedia
Many people are familiar with the Glutes and the Piriformis and often blame these muscles for any discomfort in that area. However, another deep hip rotator can be responsible for pain in the butt - Quadratus Femoris.
Quadratus Femoris is one of the Deep Six muscles. In the image, we can see how small yet thick it is, as well as "deep down there." It attaches to the sits-bone and the inside of the hip bone. Its action is to rotate the hip out.
Many yoga practitioners overstretch their QF and complain of pain here that only seems to get worse with further stretching. Quadratus Femoris can be injured during a yoga class when a practitioner sits cross-legged and bends forward too deeply or for too long. This movement (also found in other yoga poses like Pigeon pose) pulls at the attachments of QF and may potentially tear the muscle.
Due to its small size and easy accessibility of Quadratus Femoris, it is actually a fairly easy muscle to treat with massage. Any strained or pulled fibers can be treated with cross-fiber friction and trigger point release.
Another critical component of healing Quadratus Femoris is strengthening this muscle. Ironically, most people try to stretch it when it feels hurt, which re-tears the attachments and prevents healing.
Daily hip extension exercises can help build strength in QF very quickly. Ask a personal trainer or PT about hip extension with added hip external rotation like Donkey kicks, Prone leg lifts, Plank leg lifts, and elastic band exercises.