What Causes A Bruise On The Breast?

I have discovered a small bruise (about 1cm square) about a centimetre from my areola on my right breast. It is not sore & I have not experienced any kind of trauma for this to happen. I noticed it 3 days ago. I have noticed that it does blanch if pressed, which I did not think happened with bruises. I don't bruise easily and if I do have one, it's usually gone within the week. I plan to attend my GP if it is still there in a week or two, I was wondering if I should go sooner?