Good Morning and welcome to Just answer. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
There is a chance that this dental floss could cause an accordion effect in your cats intestinal tract which could be very serious. Since you are seeing it come out Flora's anus, give her a dose of Laxatone or Petromalt every 4 hrs until the remainder of the floss comes out. This will help this slide through her tact without getting stuck in an area of the tract. This can be purchased at the pet store.
Do not pull on this floss and if it gets stuck here or Flora starts vomiting and stops eating, take her to your vet. Chances are good that this will slide through but rarely, it can get stuck and then this is serious.