Hi, thanks for using PetCoach!
Poor Billy! It depends on the dose of Tylenol ingested. You can call the Pet Poison hotline to see if you know the number of pills and milligrams of the pills. Otherwise he should be taken to an open emergency vet right now. Cats can be saved from Tylenol toxicity if immediate action is taken. Sadly cats can not metabolize Tylenol and for Billy's weight of 10 lbs only 45 mg is toxic. Signs of toxicity are trouble breathing, vomiting, grey-brown gums, swollen face, neck or legs.
If you aren't sure he ate Tylenol he could be having diarrhea for many reasons and if he feels urgency and is uncomfortable he could be vocalizing.
I highly recommend taking him to a Vet now and best case he has tummy upset from something else.
Good luck!
http://www.petpoisonhelpline.com/