The ischial tuberosity is the bone in the pelvis which we sit on. It is also where the hamstrings muscles arise from the pelvis. In adolescent athletes, but also in young adults this bone can be pulled off by a forceful contraction of the hamstrings. This is called an avulsion injury.
There will be a sudden pain deep in the lower part of the buttock during sport. It will usually stop you from continuing. It is common in football and sprinting.
You should stop, and not try to carry on, then start initial treatment of rest, ice and anti inflammatories. You need to seek the attention of a sports orthopaedic specialist promptly as this injury is often missed by other doctors.
If the avulsed piece of bone is still close to the pelvis then these injuries will heal on their own with several weeks on crutches. If the bony fragment is significantly displaced then surgery may be required. Mr Willis-Owen will be able to give you more information after you have had X-rays and an MRI scan.