I have ugly stains after a double bypass. How do I know what it is?

It is hard to determine without photos and in reality without a proper exam difficult to make an accurate diagnoses. However its likely there was some bleeding under the skin on the sides of your sternum or bruising. the majority of the time the blood is simply reabsorbed and recycled but sometimes a little stays behind and oxidizes into a brownish stain of hemosiderian ( a chemical in the blood) Im not sure of how long ago your surgery was but in time most of these stains disappear but if not you can get nice results even clearing using a Q switched laser (YAG) the same one used for tattoo removal. It will break up the hemosiderian and your body will absorb it. I find 1-3 sessions is enough. See if there are any doctors in your area with this device.