A linear foreign body can get caught under the tongue or further down the gastrointestinal tract. As the intestine tries to move its contents along, it can bunch up around the string and, at worst, be damaged or perforated. Early symptoms can be subtle but often develop into vomiting, loss of appetite, lethargy and abdominal pain.
If you see a thread under the tongue or string sticking out of the mouth or bottom, do not pull on it. It may be caught further in. The vet may need to examine the mouth, take X-rays or ultrasound and decide whether the object can be removed without an operation or whether surgery is needed.
Contact a vet as soon as you know or suspect that your cat has swallowed string, yarn, ribbon, dental floss or something similar. Do not wait for obvious symptoms, as the damage can get worse while the object stays in place.