How unnerving Sophie. I'm afraid after Raven I'm super paranoid about everything, but I do truly believe it looks like what Kari mentioned, a hystiocytoma. Look at the photo on this page http://en.wikipedia.org/wiki/Histiocytoma_(dog) It really looks like it doesn't it? I'd just do what you are doing, watch it closely. Raven's cancer changed FAST and grew daily. I think it will be quickly obvious if it seems like anything more ominous.