Go to a vet now. Cats can't process acetaminophen (paracetamol, Tylenol, Panadol) the way people and dogs can, and even part of a tablet can be fatal.
Acetaminophen damages a cat's red blood cells so they can't carry oxygen (methemoglobinemia), and it can also harm the liver. Toxicosis is usually seen at 40 to 50 mg/kg, and some cats react at 10 mg/kg.
Signs can start within a few hours.
Ibuprofen and other anti-inflammatories are also much more dangerous for cats than for dogs. Never give a cat a human painkiller without a vet's advice.
US & Canada, a fee may apply. Outside North America, call your nearest emergency vet.