lump below chest?

My little boy is almost 12 weeks old, born 3 weeks early.. :-) I noticed a lump on his chest a few weeks ago and been back and fore docs.. one said it was a rib and she couldn't see what I was talking about?! The others said it could be a coiled up vessel!? Any one else had anything like this on their little ones? It shows up more in dim lighting and when it's a bit colder (when he gets out of the bath)