What Is The Painful Dent Just Above The Buttocks Visible Overnight?

I am 38 years old, 2 children, with a history of very painful SI problems. Over the last month I have developed a problem with my left knee swelling without having injured it and have had it drained twice. No RA per 2 blood tests. Today, I awakened to find a dent on the lower left side of my back, below the waist and above the cheek. It is painful but it is not red or swollen. The dent is about the size of a silver dollar and approximately 1/2 deep. Please advise. Thanks. nj2