Has your kitty vomited? Is she eating, drinking, and going to the bathroom normally? Does she gag or cough?
You will want to keep a very close eye on her. She may pass the yarn. If you notice any vomiting, diarrhea, inappetance, fever, lethargy, or gagging, she should see a vet right away. These are signs that she may have an obstruction from the yarn. Check the litterbox to see if the yarn passes. If it doesn't in a few days and you are sure she ate it, it might not be a bad idea to have an x-ray taken of her abdomen. If the yarn is stuck in the intestines and is not causing a blockage, it may still cause a perforation (tear in the intestine). If you see some of the yarn coming from the rectum, do not pull it out. Just cut off what is hanging out, and keep an eye out for more. Make sure you keep track of what she passes to be sure it is all out. The concern is that as the yarn passes through the GI system, it will get hung up or bunched up and cause obstruction. You will need to watch her closely for any of the signs I mentioned earlier. Hope this helps.