I Have A Bruise On My Buttocks That Is Really

I have a bruise on my buttocks that is really painful and grows each day. It's been about 3 days and it now has a white head in the middle of a lump. I've tried ice and heat to limited benefit. I cannot sit or lie down on my sides or back.
I am 21, generally in good health- though a little over weight. I did not have an injury leading to this pain.