Ischiogluteal bursitis happens when the bursa (a fluid filled-like sac or grease under your hamstrings) allows adjacent tissues slide over one other and cause irritation, inflammation, or damage to that region. This can occur when a traumatic injury (micro or macro trauma) happens like falling hard on a surface, this can damage the blood vessels within the ischilogluteal bursa causing reginal bleeding. If not treated the bursa will be inflamed and swollen. This generally causes an ache in the buttocks and can have sharp pains down the outside of the thigh when moving the hip joints such as walking, running, and stair climbing. When these actions occur, you should see a sports physiotherapist to not prolong the injury.