Rat has swollen spot otherwise healthy

Question
Chip Douglas has a big swollen area in the pit of his right forearm.  He does not act like it is sore and he is just as active as always.  What is it and how can I help him?

Answer
How fast did this come up?  If it doesnt come to a head in a few days it is probably a mammary tumor  Males can get them too but they usually dont return once removed like they do with females.  

Do you have a good exotic vet? This is who you will need if it ends up being a tumor.

Is there a scab on it or a pit in the center?