What does raising butt in air mean?

[Zompter]

Petunia has started raising her butt up in the air recently. It's tail up all the way ,and has only started doing it since she was introduced to another Chinchilla (also female, confirmed by the vet). She is acting absolutely normal besides this; eating drinking playing. Just haven't noticed it before. Does anybody know what it means? Could it be a dominance or mating behavior?

 

[keiren]

In my very limited experience my girl luna did this whilst she was going through heat etc and stopped a soon as she was not. So it's possibly something like that I'm hoping one of the more experienced members can shed better light on this.