Vomiting in cats can be caused by problems in the gastrointestinal tract itself, such as inflammation, food reactions or a foreign object. It can also be a sign of disease elsewhere in the body. That is why there is a big difference between a cat that vomits once and is then completely unaffected, and a cat that vomits regularly or starts to change in weight, appetite or behaviour.
Try to note how often your cat vomits, whether it happens around meals and whether your cat is still eating, drinking and using the litter tray normally. Recurring vomiting together with weight loss, diarrhoea, increased thirst or reduced appetite gives the vet important clues. Depending on the situation, the investigation may include an examination, blood tests, urine tests and imaging.
Contact a vet if the vomiting continues, or if your cat becomes lethargic, stops eating, is in pain or cannot keep water down. A cat whose general condition is clearly affected should be seen sooner.