Big lump on Guinea Pigs bottom.

Question
Hi.  I have a 2 year old male guinea pig (Goober) who developed a lump the size of a golf ball on his rump area.  It is not painful but appears to be getting slightly larger as time goes by.  Do you have any thoughts about what to do about it or what it might be?

Answer
Well, since he's two years old i know it's not his testicles, but is the hard lump in or around his testicles? It could be a cyst if it's hard. I would call a vet and see if there is anything that can be done or is it something serious to worry about.