Unusual (probably) bruising?

I wasn't sure what section to post this in, but recently I've discovered a large bruise looking thing on my chest. I know I haven't hit it on anything, nor have I been wearing anything to cause stress on my chest, but I'm unsure as to what it might be and how it got there. I've been getting random purple spots over my skin in tender places for a while and I just dismiss them, but there are a few in this 'bruise.' Can anyone maybe help tell me what it is?