What would cause a bruise on my breast?

On examining my breast after getting out of the shower, I noticed that there was a bruise about the size of a quarter or a bit bigger on the outside of my right breast just about 2" above where my bra band rests. It isn't sore to the touch. It's not solid or hard. Just looks like a fresh bruise. I don't remember being bumped or hit hard in that area or carrying anything heavy that would cause bruising. Do you think I should wait and see it it goes away or is this something that I might need to have checked out by a doctor?