Is A Hard Bump And Purple Bruise On A Buttock A Cause For Concern?

                                                       A few weeks ago, late at night, I was getting undressed for bed. I felt a hard lump in the middle of my left buttock. I looked in the mirror and I had a large deep purple bruise. The size of my hand. I did not recall hitting anything to cause such a bruise. The bruise is now fading with an outline and I still have the lump. It is not painful and only is uncomfortable if I push on it. Anything to be concerned about?