I am sorry Issy is unwell. It does sound as though she is in some discomfort, so if she is lethargic, collapsed or continuing the vomit (especially water) she will need to be seen by a vet as she may well need some further testing/pain killers or fluids. It maybe she has a simple upset tummy, in which case she should be starved for 24 hours then fed a bland diet such as white fish a rice (feed 75% fish and 25% rice) this is as long as she has free access to water and she is drinking the water without vomiting it back up.
It maybe she has developed pancreatitis as the creamy source may have been a bit rich for her, this can be serious a requires anti sickness medicines and pain killers.
It may be nothing to do with her diet and any sudden vomiting in an older cat can be a sign of organ disease, particularly the kidneys so if it isn't improving, your vet may want to run some blood tests on her.
So, to recap. If she is alert and bright and walking around OK and keeping water down/drinking then you could starve her and offer a bland diet for a few days and continue this also long as she is getting better. You could try giving her some zantac (ranitidine) to settle her stomach give her 2.5mg per kg bodyweight twice a day.
If she is colllapsed, not drinking, clearly in pain then she should be seen by your vet.
Good luck and best wishes