Lump behind guinea pig ear

Question
Hi, I live in the UK and bought two guinea pigs (male) approximately 5 weeks ago.  I have noticed while putting them in the run this morning, that Squeek has a large lump at the bald spot behind his ear.  It has literally developed overnight and is probably just under 2cm in diameter. I have made an appointment at the vet for Wed (it is Sat here) but am still a little concerned about this.  Any ideas what this could be?  Thanks, Stacey.

Answer
Hello Stacey,
the lump could be a cyst or abscess. Is your guinea pig eating and drinking okay? He should be okay if he's eating and drinking and acting normal. I am glad you are taking him to a vet because it's better to be on the safe side, because a little bump could turn into something much worse i'f it's something serious.