Looks like either fur or carpet strands. I wouldn't sweat the pellets as long as she is eating hay and greens...and drinking water. What "hairball" treatment did you use? I would suggest pushing the hay and making sure she is properly hydrated. Most buns coming out from anesthesia are already dehydrated....made worse by the fact that a lot of vets do not administer fluids during the procedure. And most of them get worse over the next couple of days since they don't usually eat or drink much. If you don't have the capability of administering sub-q fluids..it might be a good idea to get at least one round, maybe a couple, done by your vet and then you can try to maintain the hydration orally.
Randy