Hmmm... It's doen't look reddish/irritated or like it might pop. I've seen tumors(possibly cysts) on a few fish in the past but they opened up on their own and pushed the infected mass out, and then healed over really well. Just from the pictures it doesn't look like anything is wrong. Perhaps while you were on vacation this particualar fish didn't eat from the food blocks so he might have a sunken in belly, which would make the area that's circled appear more pronounced. If it looks the same on the other side of the fish, then I would bet on him simply being a bit thin. Were the food blocks completely gone when you came home?
As a side note, it won't hurt your fish if they go without food for a week while you are on vacation. In fact, I suggest not feeding them for vacations that are a week or less. (I only say this for the more hardy fish like cichlids and such) This helps keep the water quality good while you are away, and you don't have to worry about polluting the tank with uneaten food that an electric feeder or food block would produce.
