Hi Doctor, I Am Having A Lump In My Right

Hi doctor, I am having a lump in my right buttock cheek for the last few years. It used to be very small *less than one inch diameter) and not very much visible outside - I could only feel it with my hands and there was no pain. But for the last one week, it has grown almost double or triple the original size (around 2 inches diameter now) and now it is visible as a small bulge on the surface and it is also painful. What could be the cause of this?