TIP 1
Go to the vet at the slightest doubt or the slightest change in your cat's behaviour.
A cat won't clearly show pain or illness, but will instead display (sudden) unusual or different behaviour such as playing less, drinking more, eating less/more/not at all, hiding more, becoming aggressive, spraying, urinating outside the litter box, hissing at other cats, scratching more, etc.
TIP 2
If you suspect your cat is in pain or feels unwell and the vet finds no reason for it, it's possible to give a preventive painkiller to see if this affects your cat's behaviour. Discuss this with your vet.
TIP 3
A physical exam alone isn't enough to determine whether your cat is ill or in pain. Also ask the vet for a blood and urine test, and possibly an additional ultrasound. Only then can it be determined whether something is wrong with your cat.
TIP 4
Get a baby scale and weigh your cat every week by giving her a treat on the scale. Keep track of her weight in a notebook. A cat that loses or gains weight, even just 100 grams, should be examined by a vet and given an action plan accordingly.
2. Safety First!