Phoenix's lump.

I discovered this on Phoenix left hand side, right over his last 2 ribs last night. I am seriously hoping he's simply popped a rib, or run into something and has some fluid built up. It's grown in size over night, and he seems a little down. We've got the first available appointment at the vets this afternoon, and I'm terrified it's cancerous or something sinister. I am also horribly upset that I didn't notice it sooner - I'm trying to tell myself there's no way he could have had a lump for longer than about 5 minutes, because I'm always patting him, giving him massages and generally adoring him, and I would have noticed it, right?