swollen nipple

Question
Our male guinea pig is almost 4 years old. While grooming him this evening we discovered that one of his nipples is enlarged and hard. It is about the size of an unshelled peanut. He is otherwise healthy. What do we do?  

Answer
Hi Monica.  I can't possibly imagine what he has done to injure his teat like that.  Is there no other indication besides the swelling that he is injured?  Like redness, maybe a bite mark (not sure he'd bite himself, but you never know), or anything like that?  Maybe a small part missing?  I am really unsure on this one, hopefully after you let me know what else you see I can offer better advice!
Anngela