Guinea pig lump

Hello, i have 7 guinea pigs; 4 girls and 3 boys. The boy's live separate. One of my boys, buzzbee, has a strange lump on his hind quarters....It is under the skin in a perfect circle shape. It is also quite hard.He has had it for about 2 weeks now but is still eating and drinking and acting normally. I have no idea how old he is and it is neither decreasing or growing in size! i have done some research and i think it could be a build up of the scent spray they use to mark their territory, it would make sense soon as their are alot of girls near him. please help! can anyone tell me if it is dangerous?? Thank you.
   
Meg.