What Causes Bruise On Butt Spreading To Other Parts?

I woke up New years day... I blacked out after 2 drinks I am used to drinking..I've never Blacked out and neither has My best Girlfriend. Neither of us can remember anything past 11:45. I woke up with a HUGE bruise on my butt and several bruises elsewhere. None were like the big bruise.. I brushed it off.. but its getting Really Big and really really dark. How long should I wait before getting.. A bruise on my butt checked out?