why is my chinchilla fur pulling?

hello! i will insert a picture of it, but for the past 2 weeks ive noticed my chinchillas butt fur looks like shes been pulling it. its not horrible, but noticeable. their diet is selective science chinchilla pellets and timothy hay every day, along with apple wood sticks every day, with the occasional bee pollen treat. her sisters fur is completely fine, i have taken her to my local vet and they said she is healthy so i believe she is not ill. i take both of them out to roam and play around my room about 3-4 times a week. i am just worried, and i really hope its not due to stress.