According to Cornell University, College of Veterinary Medicine site, Hyperesthesia is an extreme sensitivity in an area of a cat’s skin, almost always on the back, and often in the area right in front of the tail. Anxiety and stress seem to add to a cat’s hyperesthetic reaction.
Cha-san could have some anxiety and stress.
He came out from his room in the morning, sniffed around the cat tree, and went back to his room.
He started chasing and biting his tail right after that..
He seemed okay after that all day long.
In fact, he was very close to Mochi tonight, only a foot away.
Thanks Patricia for your advice.