This condition is caused by inflammation where the hamstring tendons arise from the pelvis deep in the buttock. It is usually an overuse injury. Repetitive forceful use of the hamstrings muscles can lead to microscopic tears in the tendon when these are being formed faster than they can be repaired the problem arises.
It causes pain deep in the lower part of the buttock. It typically develops gradually, initially only being felt during and after exercise. There may be stiffness or tightness in the hamstrings. If these early signs are not recognised, then with time the pain becomes more intense and more frequent until it reaches a stage where it is present all the time and can interfere with your performance. It can cause pain sitting down, especially on hard surfaces.
If you think you have this injury it is well worth getting it checked out by Mr Willis-Owen. In the meantime, you should avoid any aggravating activities which may prolong the problem. It generally won't get better on its own. The first thing to do is to reduce your activity and start gentle hamstring stretching.
Treatment involves first confirming the diagnosis which may involve ultrasound or MRI scans, then establishing the underlying cause. Options may include activity modification, physiotherapy, orthotics and anti-inflammatories. Shock wave treatment is very good for this.