That sounds like an ear infection and is totally treatable as long as he gets vet care asap. Get him to a vet experienced with small rodents such as degus.
Well, no infection is random but it probably wasn't anything you did. He could have gotten a small scratch on his ear that got infected or something. The vet will hopefully give him a good check-up; sometimes getting an ear infection is a sign that something else is wrong (but not always so don't worry).thx. I thought he feel and twisted his neck or something. But how could he have gotten the ear infection? Is it random or does something trigger that?