Bump at the base of the tail

[BrendaM]

A few years ago we noticed a bump at the base of Angel's Tail. It is a hard bump. It does not seem to bother her. Tonight I notice the same kind of bump at the base of Zoe's Tail. The only different is Zoe's bump is soft. We pushed on it and it did not seem to bother her. Has anyone else seen these on their chinchillas and may know what it is?

Brenda

 

[Anna]

Is it the underside?