Sorry to hear your kitty is unwell, take good care of kitty, keep kitty warm and plenty of fluids as you would for a human being with the flue, and yes perhaps you need to take kitty back to the vet, if there is no change in kitty's condition.

Did you have kitty vaccinated, if not make sure you do next time, although they can still get cat flue even with the vaccine, it is much milder and not as common, flue can be serious for a cat,especially an older one, just as with humans.

Remember kitty will be miserable, as you are with flue, so give kitty lots of love and attention and the best care possible.

Good luck, and keep us posted how kitty is doing? ok