What Causes Bruises In The Dimples Of Lower Back?

Hello. I have just noticed two bruises in the dimples of my lower back. Noticed a sore lump there yesterday in that area and today is bruised. I don't remember any heavy lifting or blunt trauma that would cause this, could this be a sign of a kidney infection or something? I also have a history of prostate infections, even though I am only 31. I am 5'8" and weigh 150lbs.