My rat has a big growth?

Between last night and tonight, my rat Frosty developed some sort of a growth behind and below his right ear. He's between 2-3 years old. It's about the size of a dime, and it's basically like a giant red scabbed pimple. It's raised up on skin, and it's dark red and really hard. It doesn't seem to bother him at all, he runs around and eats like normal, and when I touch it it doesn't hurt. What could it be?