Boob bruise!

Weird question. A couple days ago, I noticed a bruise on my left boob. It didn't hurt and there wasn't any 'trauma' to the area. The girls have been pretty sore, so I would have known if something had happened. It's on the top of my boob, too, so it wouldn't have been able to be bumped on anything. And since they've been sore, they haven't been active participants in sexy time. Anyways, the bruise looks even worse today, still no pain where the bruise is, but it's nasty and bigger than a quarter. Any idea what could be causing this, excluding trauma to the area? I'm just worried it might be a symptom of something. I hope not!

Here's a pic from a few days ago and a pic from today. Super weird.