That looks like a growth or a reaction to a bite of some kind, perhaps. It certainly warrants a closer look and a vet visit is probably a good idea. Keep a close eye on it and take frequent photos to monitor any changes to the area. You might want to put some plain neosporin on it (without pain reliever) as a precaution, but it doesn't look like there is an open wound so I'm not sure if that would be necessary or helpful. I'd have a vet take a look, especially if it seems to be getting larger or spreading.