I noticed that one of my guinea pigs has a bald spot on his back. Hes had a tiny little spot in the same place since I got him, but all of a sudden it got a lot bigger. There are little stubs of hair on the spot like he bit them off. My other pig is kind of mean sometimes so i dont know if he was biting him. Any ideas of what it is? I;m guessing mites from what i read but im not sure.