This is the inflammation of a bursa (fluid filled sac) located between the hamstring tendon and the ischia tuberosity (the bones that you sit down on). It occurs with irritation of the bursa which can be triggered by a direct blow such as a a fall onto the bottom, or from repetitive trauma in sports over use injuries, or prolonged sitting on a hard surface (known as weavers bottom). Tight muscles around the area can also contribute.
It causes a dull ache in the buttock which can become sharp with movements. It can be aggravated by walking, running or climbing stairs, and particularly uncomfortable with prolonged sitting.
If you think you have this injury it is well worth getting it checked out by Mr Willis-Owen. In the mean time you should avoid any aggravating activities with may prolong the problem. It generally wont get better on its own. The first thing to do is reduce your activity and start gentle hamstring stretching.
Treatment involves first confirming the diagnosis then establishing the underlying cause. Options may include activity modification, physiotherapy, anti inflammatory. An injection under ultrasound guidance is often curative, but you need to be assessed to make sure this is right for you.