What Causes A Pus Filled Bump On The Lower Back?

For 2 days now, I've had a big Zit or whatever it's called at the bottom of my back, beginning of my ass, and it was really big last night. I blew it up... and lots of white came out... then some blood, that is where i stopped. Now it still hurts when i sit or move... and i am unsure if i should see a Doctor or not... which is why I am asking online now... I have a link to a picture of what it looks like today. I am very clean, so i am really lost as to why I have this. Any help would be appreciated.

Her eis the picture link : WWW.WWWW.WW
Thanks.