Fairly Large Growth

Question
Ok, I have had my guinea pig for about 4 years now and since he was about 3 1/2 years old I have noticed a growth on his right nipple. I didn't pay any attention to it at first because it wasn't that big, but now it has gotten bigger and his nipple is deformed looking. His water bottle water has also been brown and very smelly even though I change it everyday. What do you think is wrong and is this something that I should be concerned about?

Answer
With age comes lumps and bumps. :)

Have a vet give it the once over to make sure it's not cancer or a absesse. If it's just a harmless cyst and if it doesn't seem to cause him any pain, then i wouldn't worry about the lump too much. Keep an eye on it though once you know for certain.

As for the water bottle, I'd say thrown it out and get him a brand new one as it's full of algae, which won't make the water taste very nice.